Quantum dots of group IV semiconductor materials

ABSTRACT

The invention relates to a quantum dot. The quantum dot comprises a core including a semiconductor material Y selected from the group consisting of Si and Ge. The quantum dot also comprises a shell surrounding the core. The quantum dot is substantially defect free such that the quantum dot exhibits photoluminescence with a quantum efficiency that is greater than 10 percent.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication Ser. No. 60/309,898, filed on Aug. 2, 2001, U.S. ProvisionalApplication Ser. No. 60/309,905, filed on Aug. 2, 2001, U.S. ProvisionalApplication Ser. No. 60/309,979, filed on Aug. 2, 2001, U.S. ProvisionalApplication Ser. No. 60/310,090, filed on Aug. 2, 2001, and U.S.Provisional Application Ser. No. 60/310,095, filed on Aug. 2, 2001, thedisclosures of which are incorporated herein by reference in theirentirety.

FIELD OF THE INVENTION

[0002] This invention relates generally to quantum dots. Moreparticularly, this invention relates to quantum dots of Group IVsemiconductor materials.

BACKGROUND OF THE INVENTION

[0003] Over the past several years, there has been an increasinginterest in exploiting the extraordinary properties associated withquantum dots. As a result of quantum confinement effects, properties ofquantum dots can differ from corresponding bulk values. These quantumconfinement effects arise from confinement of electrons and holes alongthree dimensions. For instance, quantum confinement effects can lead toan increase in energy gap as the size of the quantum dots is decreased.Consequently, as the size of the quantum dots is decreased, lightemitted by the quantum dots is shifted towards higher energies orshorter wavelengths. By controlling the size of the quantum dots as wellas the material forming the quantum dots, properties of the quantum dotscan be tuned for a specific application.

[0004] Previous attempts at forming quantum dots have largely focused onquantum dots of direct band gap semiconductor materials, such as GroupII-VI semiconductor materials. In contrast to such direct band gapsemiconductor materials, Group IV semiconductor materials such as Si andGe have energy gaps, chemical properties, and other properties thatrender them more desirable for a variety of applications. However,previous attempts at forming quantum dots of Si or Ge have generallysuffered from a number of shortcomings. In particular, formation ofquantum dots of Si or Ge sometimes involved extreme conditions oftemperature and pressure while suffering from low yields and lack ofreproducibility. And, quantum dots that were produced were generallyincapable of exhibiting adequate levels of photoluminescence that can betuned over a broad spectral range. Also, previous attempts havegenerally been unsuccessful in producing quantum dots of Si or Ge thatare sufficiently stable under ambient conditions or that can be madesufficiently soluble in a variety of matrix materials.

[0005] It is against this background that a need arose to develop thequantum dots and methods for forming quantum dots described herein.

SUMMARY OF THE INVENTION

[0006] In one innovative aspect, the present invention relates to aquantum dot. In one embodiment, the quantum dot comprises a coreincluding a semiconductor material Y selected from the group consistingof Si and Ge. The quantum dot also comprises a shell surrounding thecore. The quantum dot is substantially defect free such that the quantumdot exhibits photoluminescence with a quantum efficiency that is greaterthan 10 percent.

[0007] In another embodiment, the quantum dot comprises a core includinga semiconductor material Y selected from the group consisting of Si andGe. The quantum dot also comprises a ligand layer surrounding the core.The ligand layer includes a plurality of surface ligands. The quantumdot exhibits photoluminescence with a quantum efficiency that is greaterthan 10 percent.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] For a better understanding of the nature and objects of theinvention, reference should be made to the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

[0009] FIGS. 1(a), 1(b), 1(c), and 1 (d) illustrate quantum dotsaccording to some embodiments of the invention.

[0010]FIG. 2 illustrates the energy gap of quantum dots fabricated fromsilicon plotted as a function of the size of the quantum dots, accordingto an embodiment of the invention.

[0011]FIG. 3 illustrates photoluminescence (PL) spectra from six sampleswith different sizes of silicon quantum dots, according to an embodimentof the invention.

[0012]FIG. 4(a) illustrates the energy gap of quantum dots fabricatedfrom germanium plotted as a function of the size of the quantum dots,according to an embodiment of the invention.

[0013]FIG. 4(b) illustrates size-selective photoluminescence (PL)spectra for different sizes of germanium quantum dots, according to anembodiment of the invention.

[0014]FIG. 5(a) illustrates concentration dependence of the linear indexof refraction of engineered nonlinear nanocomposite materials doped withsilicon and germanium quantum dots, according to an embodiment of theinvention.

[0015]FIG. 5(b) illustrates concentration dependence of the opticalnonlinearity of engineered nonlinear nanocomposite materials doped withsilicon and germanium quantum dots, according to an embodiment of theinvention.

[0016] FIGS. 6(a), 6(b), 6(c), 6(d), 6(e), and 6(f) illustrate nonlineardirectional couplers comprising engineered nonlinear nanocompositematerials, according to some embodiments of the invention.

[0017] FIGS. 7(a), 7(b), 7(c), 7(d), 7(e), and 7(f) illustrate anembodiment of a nonlinear Mach-Zehnder (MZ) interferometer comprising anengineered nonlinear nanocomposite material.

[0018] FIGS. 8(a), 8(b), 8(c), and 8(d) illustrate an alternativeembodiment of a nonlinear MZ interferometer comprising an engineerednonlinear nanocomposite material.

[0019]FIG. 9 illustrates a figure-of-merit (FOM) for all-opticalswitching with an engineered nonlinear nanocomposite material as afunction of quantum dot size, according to an embodiment of theinvention.

[0020] FIGS. 10(a) and 10(b) illustrate photoluminescence spectra ofsilicon quantum dots made in accordance with an embodiment of theinvention.

[0021] FIGS. 11(a) and 11(b) illustrate photoluminescence spectra ofgermanium quantum dots made in accordance with an embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION Definitions

[0022] The following definitions may apply to some of the elementsdescribed with regard to some embodiments of the invention. Thesedefinitions may likewise be expanded upon herein.

[0023] As used in this specification and the appended claims, thesingular forms “a an”, and “the” include plural references unless thecontent clearly dictates otherwise. Thus, for example, reference to “aquantum dot” includes a mixture of two or more such quantum dots and mayinclude a population of such quantum dots.

[0024] “Optional” or “optionally” means that the subsequently describedevent or circumstance may or may not occur and that the descriptionincludes instances where the event or circumstance occurs and instancesin which it does not. For example, the phrase “optionally surroundedwith a shell” means that the shell may or may not be present and thatthe description includes both the presence and absence of such a shell.

[0025] Embodiments of the invention relate to a class of novel materialscomprising quantum dots. As used herein, the terms “quantum dot”, “dot”,and “nanocrystal” are synonymous and refer to any particle with sizedependent properties (e.g., chemical, optical, and electricalproperties) along three orthogonal dimensions. A quantum dot can bedifferentiated from a quantum wire and a quantum well, which havesize-dependent properties along at most one dimension and twodimensions, respectively.

[0026] It will be appreciated by one of ordinary skill in the art thatquantum dots can exist in a variety of shapes, including but not limitedto spheroids, rods, disks, pyramids, cubes, and a plurality of othergeometric and non-geometric shapes. While these shapes can affect thephysical, optical, and electronic characteristics of quantum dots, thespecific shape does not bear on the qualification of a particle as aquantum dot.

[0027] For convenience, the size of quantum dots can be described interms of a “diameter”. In the case of spherically shaped quantum dots,diameter is used as is commonly understood. For non-spherical quantumdots, the term diameter, unless otherwise defined, refers to a radius ofrevolution (e.g., a smallest radius of revolution) in which the entirenon-spherical quantum dot would fit.

[0028] A quantum dot will typically comprise a “core” of one or morefirst materials and can optionally be surrounded by a “shell” of asecond material. A quantum dot core surrounded by a shell is referred toas a “core-shell” quantum dot.

[0029] The term “core” refers to the inner portion of the quantum dot. Acore can substantially include a single homogeneous monoatomic orpolyatomic material. A core can be crystalline, polycrystalline, oramorphous. A core may be “defect” free or contain a range of defectdensities. In this case, “defect” can refer to any crystal stackingerror, vacancy, insertion, or impurity entity (e.g., a dopant) placedwithin the material forming the core. Impurities can be atomic ormolecular.

[0030] While a core may herein be sometimes referred to as“crystalline”, it will be understood by one of ordinary skill in the artthat the surface of the core may be polycrystalline or amorphous andthat this non-crystalline surface may extend a measurable depth withinthe core. The potentially non-crystalline nature of the “core-surfaceregion” does not change what is described herein as a substantiallycrystalline core. The core-surface region optionally contains defects.The core-surface region will preferably range in depth between one andfive atomic-layers and may be substantially homogeneous, substantiallyinhomogeneous, or continuously varying as a function of position withinthe core-surface region.

[0031] Quantum dots may optionally comprise a “shell” of a secondmaterial that surrounds the core. A shell can include a layer ofmaterial, either organic or inorganic, that covers the surface of thecore of a quantum dot. A shell may be crystalline, polycrystalline, oramorphous and optionally comprises dopants or defects. The shellmaterial is preferably an inorganic semiconductor with a bandgap that islarger than that of the core material. In addition, preferred shellmaterials have good conduction and valence band offsets with respect tothe core such that the conduction band is desirably higher and thevalence band is desirably lower than those of the core. Alternatively,the shell material may have a bandgap that is smaller than that of thecore material, and/or the band offsets of the valence or conductionbands may be lower or higher, respectively, than those of the core. Theshell material may be optionally selected to have an atomic spacingclose to that of the core material.

[0032] Shells may be “complete”, indicating that the shell substantiallycompletely surrounds the outer surface of the core (e.g., substantiallyall surface atoms of the core are covered with shell material).Alternatively, the shell may be “incomplete” such that the shellpartially surrounds the outer surface of the core (e.g., partialcoverage of the surface core atoms is achieved). In addition, it ispossible to create shells of a variety of thicknesses, which can bedefined in terms of the number of “monolayers” of shell material thatare bound to each core. A “monolayer” is a term known in the artreferring to a single complete coating of a shell material (with noadditional material added beyond complete coverage). For certainapplications, shells will preferably be of a thickness betweenapproximately 0 and 10 monolayers, where it is understood that thisrange includes non-integer numbers of monolayers. Non-integer numbers ofmonolayers can correspond to the state in which incomplete monolayersexist. Incomplete monolayers may be either homogeneous or inhomogeneous,forming islands or clumps of shell material on the surface of thequantum dot. Shells may be either uniform or nonuniform in thickness. Inthe case of a shell having nonuniform thickness, it is possible to havean “incomplete shell” that contains more than one monolayer of shellmaterial. For certain applications, shell thickness will preferablyrange between approximately 1 Å and 100 Å.

[0033] It will be understood by one of ordinary skill in the art thatthere is typically a region between the core and shell referred toherein as an “interface region”. The interface region may comprise anatomically discrete transition between the material of the core and thematerial of the shell or may comprise an alloy of the materials of thecore and shell. The interface region may be lattice-matched or unmatchedand may be crystalline or noncrystalline. The interface region maycontain one or more defects or be defect-free. The interface region maybe homogeneous or inhomogeneous and may comprise chemicalcharacteristics that are graded between the core and shell materialssuch that a gradual or continuous transition is made between the coreand the shell. Alternatively, the transition can be discontinuous. Thewidth of the interface region can range from an atomically discretetransition to a continuous graded alloy of core and shell materials thatare purely core material in the center of the quantum dot and purelyshell material at the outer surface. Preferably, the interface regionwill be between one and five atomic layers thick.

[0034] A shell may optionally comprise multiple layers of a plurality ofmaterials in an onion-like structure, such that each material acts as ashell for the next-most inner layer. Between each layer there isoptionally an interface region. The term “shell” is used herein todescribe shells formed from substantially one material as well as aplurality of materials that can, for example, be arranged as multi-layershells.

[0035] A quantum dot may optionally comprise a “ligand layer” comprisingone or more surface ligands (e.g., organic molecules) surrounding a coreof the quantum dot. A quantum dot comprising a ligand layer may or maynot also comprise a shell. As such, the surface ligands of the ligandlayer may bind, either covalently or non-covalently, to either the coreor the shell material or both (in the case of an incomplete shell). Theligand layer may comprise a single type of surface ligand (e.g., asingle molecular species) or a mixture of two or more types of surfaceligands (e.g., two or more different molecular species). A surfaceligand can have an affinity for, or bind selectively to, the quantum dotcore, shell, or both at least at one point on the surface ligand. Thesurface ligand may optionally bind at multiple points along the surfaceligand. The surface ligand may optionally contain one or more additionalactive groups that do not interact specifically with the surface of thequantum dot. The surface ligand may be substantially hydrophilic,substantially hydrophobic, or substantially amphiphilic. Examples of thesurface ligand include but are not limited to an isolated organicmolecule, a polymer (or a monomer for a polymerization reaction), aninorganic complex, and an extended crystalline structure.

[0036] It will be understood by one of ordinary skill in the art thatwhen referring to a population of quantum dots as being of a particular“size”, what is meant is that the population is made up of adistribution of sizes around the stated “size”. Unless otherwise stated,the “size” used to describe a particular population of quantum dots willbe the mode of the size distribution (i.e., the peak size).

[0037] As used herein, the “size” of a quantum dot will refer to thediameter of a core of the quantum dot. If appropriate, a separate valuewill be used to describe the thickness of a shell surrounding the core.For instance, a 3 nm silicon quantum dot with a 1.5 nm SiO₂ shell is aquantum dot comprising a 3 nm diameter core of silicon surrounded by a1.5 nm thick layer of SiO₂, for a total diameter of 6 nm.

[0038] For certain applications, the thickness of the ligand layer is asingle monolayer or less and can sometimes be substantially less than asingle monolayer.

[0039] As used herein, the term “photoluminescence” refers to theemission of light of a first wavelength (or range of wavelengths) by asubstance (e.g., a quantum dot) that has been irradiated with light of asecond wavelength (or range of wavelengths). The first wavelength (orrange of wavelengths) and the second wavelength (or range ofwavelengths) can be the same or different.

[0040] As used herein, the term “quantum efficiency” refers to the ratioof the number of photons emitted by a substance (e.g., a quantum dot) tothe number of photons absorbed by the substance.

[0041] As used herein, the term “monodisperse” refers to a population ofquantum dots wherein at least about 60% of the population, preferably75% to 90% of the population, or any integer or noninteger therebetween,falls within a specified particle size range. A population ofmonodispersed particles deviates less than 20% root-mean-square (rms) indiameter, more preferably less than 10% rms, and most preferably lessthan 5% rms.

[0042] “Optically pure” refers to a condition in which light passingthrough or past a material is substantially unchanged in mode quality asa result of inhomogeneities in the material or modulations at theinterface between materials. This does not include mode disruptionresulting from changes in index of refraction of waveguides. Forinstance, a material with large aggregates of quantum dots capable ofscattering light would not be optically pure. The same material withaggregates of a size that do not significantly scatter light, however,would be optically pure. It will be apparent to one of ordinary skill inthe art that what is meant above by “substantially unchanged” willdepend on the optical requirements of a particular application. To thisend, “optically pure” refers to the level of optical purity required forthe application in which the material is to be used.

[0043] “Optically homogeneous” is defined as being homogeneous across alength scale that is significant for optical waves, preferably greaterthan 250 nm, more preferably greater than 4 μm, and most preferablygreater than ˜000 μm.

[0044] A “waveguide structure” is a term of art and refers to an opticaldevice capable of transmitting light from one location to another. Awaveguide structure can transmit light through the use of guiding bylocalized effective index differences. One example of this involvestotal internal reflection within a “waveguide core”, with an index ofrefraction n₁, surrounded by a “cladding”, with an index of refractionn₂, wherein n₁>n₂. Another example of a waveguide structure involvesappropriately micro or nanostructured materials such as photonic bandgapmaterials where the guiding results from the periodic micro- ornano-structure of the materials.

[0045] “Cladding” is any material that surrounds the waveguide core in awaveguide structure such that n₁>n₂. In a typical waveguide structure,light propagates as a traveling wave within and along the length of the“waveguide core” and evanescently decays within the cladding with adecay constant related to the ratio of n₁ to n₂. Light trapped within,and traveling along, the length of a waveguide core is referred to asbeing “guided”.

[0046] The shape of a waveguide core or a cladding can typically bedescribed in terms of its “cross-section”. The cross-section is theshape created by cutting the waveguide core or the cladding along theaxes perpendicular to the longitudinal axis of the waveguide structure.The longitudinal axis is the axis in which guided light travels.

[0047] “Optical fibers” and “planar waveguides” are two common forms ofwaveguide structures known in the art. “Optical fiber”, as the term iscommonly used, typically refers to a structure comprising asubstantially cylindrical waveguide core surrounded by a substantiallycylindrical cladding and optionally comprising a flexible, protectiveouter-coating. Alternatively, or in conjunction, an optical fiber cancomprise a non-cylindrical waveguide core with a cross-section shaped asa trapezoid, a circle, an oval, a triangle, or another geometric andnongeometric shape.

[0048] “Planar waveguides” are waveguide structures fabricated on asubstrate by a variety of methods. “Planar waveguides” typicallycomprise a substantially rectangular waveguide core. Alternatively, orin conjunction, planar waveguides can comprise non-rectangular waveguidecores with cross-sections of trapezoids, circles, ovals, triangles, or aplurality of other geometric and nongeometric shapes. While the term“planar” suggests a flat structure, the term “planar waveguide”, as usedherein, also refers to structures comprising multiple flat layers.Optionally, one or more layers in a planar waveguide are not flat. Oneof skill in the art will appreciate that the key aspect of a “planarwaveguide” is that it is a waveguide structure fabricated on a“substrate”. Unless otherwise stated, the term “waveguide structure”will be used herein to describe a planar waveguide.

[0049] “Waveguide substrate” or “substrate” is used herein to describethe material on which a planar waveguide is located. It is common that aplanar waveguide is fabricated directly on the surface of the substrate.The substrate typically comprises a solid support such as, for example,a silicon wafer and optionally comprises an additional “buffer layer”that separates the waveguide structure from the solid support. Thebuffer layer optionally comprises a plurality of layers comprising oneor more materials or combination of materials. The buffer layer mayoptionally act, in part, as a cladding. Alternatively, the waveguidesubstrate may be a flexible substrate serving the same purpose.

[0050] “Single mode” waveguide structures are those waveguide structures(either planar or fiber optic) that typically support a single opticalmode (e.g., TEM00). Such waveguide structures are preferred according tosome embodiments of the invention. “Multi-mode” waveguide structures arethose waveguide structures that typically support multiple optical modessimultaneously.

[0051] “Waveguide diameter” is herein used to describe the diameter of asubstantially cylindrical waveguide core of an optical fiber. Waveguidediameter is also used to describe the diameter of a substantiallycylindrical core on a planar waveguide.

[0052] “Waveguide width” or “width” is used herein to describe thecross-sectional dimension of a substantially rectangular waveguide corethat is oriented parallel to the substrate surface. This is alsoreferred to as the “horizontal dimension” of the waveguide core.“Waveguide height” or “height” is used herein to describe thecross-sectional dimension of a substantially rectangular waveguide corethat is oriented perpendicular to the substrate surface. This is alsoreferred to as the “vertical dimension” of the waveguide core. Based onthe definitions of “width” and “height” described here, one of ordinaryskill in the art will understand the translation of these terms to othergeometrically or nongeometrically shaped waveguide cores. Unlessotherwise stated, the standard definitions of width and height used ingeometry will be used to describe geometric cross-sectional shapes.

[0053] “Core taper” refers to a region of the waveguide core in whichthe geometry of the waveguide core is changed. This may comprisechanging the size and/or shape of the waveguide core in one or twodimensions. A core taper, for example, may comprise a transition of awaveguide core with a square cross-section of 15 μm×15 μm to a waveguidecore with a square cross-section of 7 μm×7 μm. A core taper may also,for example, comprise a transition from a waveguide core with a squarecross-section of 15 μm×15 μm to a waveguide core with a circularcross-section of 10 μm in diameter. Many other forms of core-tapers arepossible and will be understood from the above definition.

[0054] A “core taper” is typically engineered to gradually change thecharacteristics of the waveguide structure over a defined distance,referred to as the “taper length”. Ideally, the taper length will belong enough so that the transition preserves the mode structure of anoptical signal through the taper. In particular, it is preferred, butnot required, that a single optical mode entering a taper remains asingle mode after exiting the taper. This retention of themode-structure is referred to as an “adiabatic transition”. While theterm “adiabatic transition” is commonly used, those of ordinary skill inthe art will recognize that it is typically not possible to have aperfectly adiabatic transition, and that this term can be used todescribe a transition in which the mode structure is substantiallyundisrupted.

[0055] A “cladding taper” is a novel embodiment disclosed herein that issimilar to a core taper; however, it refers to a change in width of thecladding around the waveguide core. Similar to a core taper, a claddingtaper can be used to change the size and/or shape of the cladding andcan be defined to have a taper length. The taper length can be such asto produce an adiabatic or nonadiabatic transition.

[0056] Both core and cladding tapers may optionally refer to the case inwhich the index of refraction of the materials in the core or claddingare gradually changed, or “graded” over the taper length. As usedherein, the term “gradually” refers to changes that occur continuouslyor in small steps over a given nonzero distance. Core and claddingtapers may optionally comprise changes to the index, size, and/or shapeof the core or cladding, respectively.

[0057] A “bend” is used herein to describe a portion of a planarwaveguide in which the planar waveguide displays a degree of curvaturein at least one dimension. Typically, the cross-section of the waveguideis substantially unchanged within the bend. Typically, bends will besmooth and continuous and can be described in terms of a radius ofcurvature at any given point within the bend. While bends can curve theplanar waveguide both parallel and perpendicular to the substrate (e.g.,horizontal or vertical bends, respectively), unless otherwise stated,the term “bend” will herein refer to horizontal bends. Optionally, bendscan also comprise tapers.

[0058] A “multimode interference device” or multimode interferometer(MMI) refers to an optical device in which the cross-section of thewaveguide core is substantially changed (typically increased) within ashort propagation length, leading to a region of waveguide core in whichmore than one mode (but typically fewer than 10 modes) may propagate.The interaction of these propagating multiple modes defines the functionperformed by the MMI. MMI devices include fixed ratiosplitters/combiners and wavelength multiplexers/demultiplexers.

[0059] As used herein, a “waveguide coupler”, “optical coupler”, and“directional coupler” are synonymous and refer to a waveguide structurein which light is evanescently coupled between two or more waveguidecores within a coupling region such that the intensity of the lightwithin each of the individual cores oscillates periodically as afunction of the length of the coupling region. A more detaileddescription of a waveguide coupler is disclosed below.

[0060] A “nonlinear waveguide coupler” is a waveguide coupler in whichthe region between and/or around two or more coupled waveguide cores isfilled with a material (e.g., an “active material”) with an index ofrefraction that can be changed. By changing the index of refraction ofthe active material, the coupling characteristics of the nonlinearwaveguide coupler can be modified. Alternatively, the active materialmay be contained within one or more of the coupled waveguide cores(e.g., as a section of one of the waveguide cores).

[0061] A “Mach-Zehnder interferometer” or “MZ interferometer” (MZI) is awaveguide structure in which light from a waveguide core (e.g., an“input waveguide core”) is split into two or more separate waveguidecores (e.g., “waveguide arms” or “arms”). Light travels a defineddistance within the arms and is then recombined into a waveguide core(e.g., an “output waveguide core”). In a MZ interferometer, the historyof the optical signals in each arm affects the resulting signal in theoutput waveguide core. A more detailed description of a MZinterferometer is disclosed below.

[0062] A “nonlinear MZ interferometer” is a MZ interferometer in whichone or more of the waveguide arms comprise an active material. Theactive material may be in the core and/or cladding of the waveguide arm.Modifying the index of refraction of the active material modulates thesignal in the output waveguide core by changing the degree ofconstructive and/or destructive interference from the waveguide arms.

[0063] “Active material” refers to any material with nonlinear opticalproperties that can be used to manipulate light in accordance with someembodiments of the invention. While the term active material willtypically be used to refer to an engineered nonlinear nanocompositematerial as described herein, the term may also be used to describeother nonlinear materials known in the art.

[0064] “Active region” refers to the region of an optical device inwhich the index of refraction of the active material is modulated inorder to manipulate light. In the case of an electro-optic modulator,the active region is that area of the device where a voltage is applied.In a χ⁽³⁾ based device, the active region is that area to which atrigger-signal is applied. Note that while the active region can be theonly region of the device in which an intentional change in opticalproperties occurs, it does not restrict the location of the activematerial, which may extend beyond the active region. Regions containingactive materials outside the active region are typically not modulatedduring normal operation of the device. “Active length” describes thelength of the active region along the longitudinal axis of the device.

[0065] In the case of optical devices employing evanescent coupling oflight between two waveguide cores (e.g., a waveguide coupler), the“interaction region” or “coupling region” is the region of the opticaldevice in which the coupling occurs. As is typically understood in theart, all waveguides can couple at some theoretically non-zero level. Theinteraction region, however, is typically considered to be that regionof the optical device in which evanescent fields of the waveguidesoverlap to a significant extent. Here again, the interaction region doesnot restrict the extent of either the active region or the activematerial, which may be greater or lesser in extent than the interactionregion.

[0066] “Interaction length” describes the length of the interactionregion. “Interaction width” is the spacing between two coupledwaveguides within the interaction region. Unless otherwise stated, theinteraction width is assumed to be substantially constant across atleast a portion of the interaction length.

[0067] “Trigger pulse”, “trigger signal”, “control pulse”, “controlsignal”, “control beam”, and “activation light” are synonymous and referto light that is used to create a transient change in the index ofrefraction in the materials of some embodiments of the presentinvention. A trigger pulse can either be pulsed or CW.

[0068] “Data pulse”, data signal”, and “data beam” are synonymous andrefer to light used to transmit information through an optical device. Adata pulse can optionally be a trigger pulse. A Data pulse can either bepulsed or CW.

[0069] “CW light” and “CW signal” are synonymous and refer to light thatis not pulsed.

[0070] “Wavelength range-of-interest” refers to any range of wavelengthsthat will be used with a particular optical device. Typically, this willinclude both the trigger and data signals, where the ranges for thetrigger and data signals can be the same or different. For instance, ifa device is fabricated for use in the 1550 nm telecom range, the datawavelength range-of-interest may be defined as 1.5 μm to 1.6 μm, and thetrigger wavelength range-of-interest may be defined as 1.5 μm to 1.6 μm(or a different range). For devices in the 1300 nm range, the datawavelength range-of-interest may be defined as 1.25 μm-1.35 μm. Whilethese are preferred wavelength range-of-interests, it will be understoodthat the specific wavelength range-of-interest can be differentdepending on the specific application. The ability to tune the materialsof embodiments of the current invention implies that any wavelengthrange-of-interest may be used. In general, 300 nm to 4000 nm is apreferred wavelength range-of-interest, more preferably 300 nm to 2000nm, more preferably 750 nm to 2000 nm, more preferably 1260 nm to 1625nm, most preferably 1310±50 nm and 1580±50 nm.

[0071] Quantum Dots

[0072] Embodiments of the current invention, in part, exploit theextraordinary properties of quantum dots. Quantum dots have optical andelectronic properties that can be dependent (sometimes stronglydependent) on both the size and the material forming the quantum dots.

[0073] In nature, it is the size range on the order of a few nanometersin which the quantum mechanical characteristics of atoms and moleculesoften begin to impact and even dominate the classical mechanics ofeveryday life. In this size range, a material's electronic and opticalproperties can change and become dependent on size. In addition, as thesize of a material gets smaller, and therefore more atomic-like, manycharacteristics change or are enhanced due to a redistribution ofoscillator strength and density of states. These effects are referred toas “quantum confinement” effects. For example, quantum confinementeffects can cause the energy gap of the quantum dot or the energy of thelight emitted from the quantum dot to increase as the size of thequantum dot decreases. These quantum confinement effects result in theability to finely tune many properties of quantum dots (e.g., opticaland electronic properties) by carefully controlling their size. Thiscontrol provides one critical aspect of some embodiments of the currentinvention.

[0074] A quantum dot will typically be in a size range between about 1nm and about 1000 nm in diameter or any integer or fraction of aninteger therebetween. Preferably, the size will be between about 1 nmand about 100 nm, more preferably between about 1 nm and about 50 nm orbetween about 1 nm to about 20 nm (such as about 2, 3, 4, 5, 6, 7, 8, 9,10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 nm or any fraction of aninteger therebetween), and more preferably between about 1 nm and 10 nm.

[0075] FIGS. 1(a), 1(b), 1(c), and 1(d) illustrates quantum dotsaccording to some embodiments of the invention. In particular, FIG. 1(a)illustrates a quantum dot 100 comprising a core 102, according to anembodiment of the invention. A core (e.g., the core 102) of a quantumdot may comprise inorganic crystals of Group IV semiconductor materialsincluding but not limited to Si, Ge, and C; Group II-VI semiconductormaterials including but not limited to ZnS, ZnSe, ZnTe, ZnO, CdS, CdSe,CdTe, CdO, HgS, HgSe, HgTe, HgO, MgS, MgSe, MgTe, MgO, CaS, CaSe, CaTe,CaO, SrS, SrSe, SrTe, SrO, BaS, BaSe, BaTe, and BaO; Group III-Vsemiconductor materials including but not limited to AlN, AlP, AlAs,AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, and InSb; Group IV-VIsemiconductor materials including but not limited to PbS, PbSe, PbTe,and PbO; mixtures thereof; and tertiary or alloyed compounds of anycombination between or within these groups. Alternatively, or inconjunction, a core can comprise a crystalline organic material (e.g., acrystalline organic semiconductor material) or an inorganic and/ororganic material in either polycrystalline or amorphous form.

[0076] A core may optionally be surrounded by a shell of a secondorganic or inorganic material. FIG. 1(b) illustrates a quantum dot 104according to another embodiment of the invention. Here, the quantum dot104 comprises a core 106 that is surrounded by a shell 108. A shell(e.g., the shell 108) may comprise inorganic crystals of Group IVsemiconductor materials including but not limited to Si, Ge, and C;Group II-VI semiconductor materials including but not limited to ZnS,ZnSe, ZnTe, ZnO, CdS, CdSe, CdTe, CdO, HgS, HgSe, HgTe, HgO, MgS, MgSe,MgTe, MgO, CaS, CaSe, CaTe, CaO, SrS, SrSe, SrTe, SrO, BaS, BaSe, BaTe,and BaO; Group III-V semiconductor materials including but not limitedto AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, and InSb;mixtures thereof; and tertiary or alloyed compounds of any combinationbetween or within these groups. Alternatively, or in conjunction, ashell can comprise a crystalline organic material (e.g., a crystallineorganic semiconductor material) or an inorganic and/or organic materialin either polycrystalline or amorphous form. A shell may be doped orundoped, and in the case of doped shells, the dopants may be eitheratomic or molecular. A shell may optionally comprise multiple materials,in which different materials are stacked on top of each other to form amulti-layered shell structure.

[0077] As illustrated in FIGS. 1(c) and 1(d), a quantum dot mayoptionally comprise a ligand layer comprising one or more surfaceligands (e.g., organic molecules) surrounding a core, according to someembodiments of the invention. In FIG. 1(c), a quantum dot 110 comprisesa core 112 and a ligand layer 114 surrounding the core 112. In FIG.1(d), a quantum dot 116 comprises a core 118 and a ligand layer 122surrounding the core 118. Here, the quantum dot 116 also comprises ashell 120 surrounding the core 118, where the shell 120 is positionedbetween the core 118 and the ligand layer 122.

[0078] Optical Properties

[0079] Linear Optical Properties:

[0080] One of the most dramatic examples of “quantum confinement”effects is that, for a semiconductor material, the energy gap shifts asa function of size. This can be seen in FIG. 2, where the energy gap ofquantum dots fabricated from silicon, referred to herein as “siliconquantum dots”, is plotted as a function of the size (e.g., diameter) ofthe quantum dots, according to an embodiment of the invention. Thesilicon quantum dots were made as described herein. The vertical axisrepresents the energy gap of the silicon quantum dots, and thehorizontal axis represents the size of the silicon quantum dots. Theobserved values for the energy gap (dots with error bars) are comparedagainst pseudopotential and tight-binding models (solid line) andagainst the simple effective mass theory (dashed line).

[0081] The same effect can be seen for the emission wavelength as afunction of the size of quantum dots. FIG. 3 illustratesphotoluminescence (PL) spectra from six samples with different sizes ofsilicon quantum dots, according to an embodiment of the invention. Thesilicon quantum dots were made as described herein and include shellsformed of an oxide. The vertical axis represents a normalized PL signal,and the horizontal axis represents the emission wavelength. The PLspectra illustrated in FIG. 3 is obtained by optically exciting thesilicon quantum dots with ultraviolet light. The wavelength of theoptical excitation is shorter than the wavelength at the absorption edgeof the silicon quantum dots. FIG. 3 demonstrates the range of sizes thatcan be made with the methods described herein. The quantum dots shown atthe top of FIG. 3 are not drawn to scale and are meant to illustrate therelative size of the quantum dots responsible for the PL spectra. FIGS.2 and 3 demonstrate the unprecedented control that can be obtained overabsorption and emission characteristics of the silicon quantum dots.

[0082] Through a series of relations called the Kramers-Kroenigequations, the properties of refractive index and dielectric constantcan be related to absorption. As such, size-dependent control ofabsorption allows control of refractive index.

[0083] In addition to the size of a quantum dot, the optical andelectronic properties are also strongly influenced by the material fromwhich it is fabricated. Quantum confinement effects represent amodulation of the bulk properties of the material. As such, any changesresulting from a reduction in size are made relative to the bulkproperties of the material. By selecting (e.g., independently selecting)the appropriate combination of quantum dot size and material, an evengreater control of the optical and electronic properties of a quantumdot is provided. As an example, FIGS. 4(a) and (b) show the sizedependent absorption and emission of germanium quantum dots, whichdiffer from those of silicon quantum dots, according to an embodiment ofthe invention. The germanium quantum dots were made as described herein.In FIG. 4(a), the vertical axis represents the energy gap of thegermanium quantum dots, and the horizontal axis represents the size ofthe germanium quantum dots. The observed values for the energy gap (opendots with error bars) are compared against theoretical predictions(solid dots and solid line). In FIG. 4(b), a size-selective PL spectrumis shown, where the vertical axis represents a normalized PL signal, andthe horizontal axis represents the emission wavelength. The far rightcurve is offset vertically for clarity. The PL spectra shown in FIG.4(b) are collected using different excitation wavelengths, such thatonly quantum dots with energy gaps less than or equal to the photonenergy of the excitation light (i.e., greater than a certain quantum dotsize) are excited.

[0084] Relation of Size and Material to Dielectric Constant and Index ofRefraction

[0085] For most materials, the index of refraction far from resonancedecreases as the energy gap of the material increases (a consequence ofthe Kramers-Kroenig equations). This explains, for example, why theindex of refraction of transparent materials (e.g., silica, metalhalides, and organics) is less than that for inorganic semiconductorswith smaller relative absorption energies. This effect also typicallyapplies to quantum dots. In this case, as the size of the quantum dotdecreases, the energy gap increases, decreasing the index of refraction.Thus, for quantum dots, the off-resonant index of refraction (at a fixedwavelength) typically correlates with size, affording another method tocontrol the optical properties of the quantum dots.

[0086] Relation of Concentration of Quantum Dots to Dielectric Constantand Index of Refraction

[0087] Embodiments of the invention involve altering the index ofrefraction of a material by varying the concentration of quantum dots inthe material. An example of this is shown in FIG. 5(a), whichillustrates concentration dependence of the linear index of refractionof engineered nanocomposite materials doped with silicon and germaniumquantum dots, according to an embodiment of the invention. The siliconand germanium quantum dots were made in accordance with the methodsdescribed herein. The index of refraction is plotted as a function ofthe quantum dot concentration expressed in weight percent. In thisfigure, the index of refraction is measured in the visible range (sodiumD line).

[0088] This concentration dependence provides yet another method ofcontrolling the overall refractive index of a material by utilizing theproperties of quantum dots. The ability to embed quantum dots into avariety of host materials will be discussed in a later section.

[0089] Nonlinear Optical Properties

[0090] In general, a wide variety of nonlinear optical phenomena canarise when materials are exposed to high-intensity light. Some of thesenonlinear phenomena are used in certain aspects of telecommunications(e.g., Raman amplifiers) and many are being considered for future use(e.g., four-wave mixing, cross-phase modulation, and solitons). Althoughnonlinear phenomena are typically associated with high-intensities,these phenomena are also observed at lower intensities due to phasematching, resonant enhancement, and/or long interaction lengths.

[0091] Light incident on a material can induce a polarization (P), whichcan be expressed as (in SI units)

P=ε ₀ χE=ε ₀└χ⁽¹⁾ E+χ ⁽²⁾ E×E+χ ⁽³⁾ E×E×E+ . . . ┘,

[0092] where E is the electric field strength, ε₀ is the electricpermittivity, χ is the overall optical susceptibility, and χ^((n)) isthe nth order optical susceptibility. Since χ⁽²⁾ phenomena are typicallyonly present in materials that lack inversion symmetry (e.g.,non-centrosymmetry), certain embodiments of the invention primarilyexploit χ⁽³⁾ phenomena, which can be exhibited by all materials. Itshould be recognized that tensor elements of χ⁽³⁾ are in general complexquantities. The induced refractive index change Δn and the nonlinearindex of refraction γ are related to the real part of appropriate tensorelements of χ⁽³⁾, e.g., Re[χ⁽³⁾ ₁₁₁₁)], while the two-photon absorptioncoefficient β is related to the imaginary part of appropriate tensorelements of χ⁽³⁾, e.g., Im[χ⁽³⁾ ₁₁₁₁]). In particular, certainembodiments of the invention exploit phenomena that change the index ofrefraction of a material by creating an effective optical susceptibility

χ_(eff)χ⁽¹⁾+χ⁽³⁾ E×E=χ ⁽¹⁾+χ⁽³⁾ I,

[0093] where I is the intensity of the particular light beam creatingthe effective optical susceptibility (and where the higher order termsare assumed to be small and are therefore neglected here, although theycan be utilized as well), which can affect the same light beam oranother light beam at the same or different frequency. This leads to aneffective or overall index of refraction given by

n(ω′)=n ₀+γ(ω′,ω)I(ω),

[0094] and an operational definition for a nonlinear index of refractionγ given by${\gamma \left( {\omega^{\prime},\omega} \right)} = \frac{{n\left( \omega^{\prime} \right)} - n_{0}}{I(\omega)}$

[0095] where n(ω′) is the effective index of refraction at ω′, n₀ is thelow-intensity refractive index (e.g., the linear index of refraction),and I(ω) is the intensity of light with optical frequency ω that createsthe effective optical susceptibility or index change. The nonlinearindex of refraction γ(ω′,ω) is related to χ⁽³⁾ _(ijkl)(−ω′, ω′, ω, −ω),e.g., χ⁽³⁾ ₁₁₁₁(−ω′, ω′, ω, −ω). If only one light beam is involved,then ω′ can be set equal to ω. If two light beams are involved, then ω′and ω can be the same or different. Situations where ω′and ω are thesame can correspond to degenerate conditions (which is further discussedherein), in which case the nonlinear index of refraction γ can bereferred to as a degenerate nonlinear index of refraction (or γ_(deg))Situations where ω′ and ω are different can correspond to non-degenerateconditions (which is further discussed herein), in which case thenonlinear index of refraction γ can be referred to as a non-degeneratenonlinear index of refraction (or γ_(nondeg)). As one of ordinary skillin the art will understand, an optical frequency of a light beam (e.g.,ω or ω′) is inversely related to a wavelength of the light beam (e.g., λor λ′).

[0096] This intensity dependent refractive index n(ω′) can be exploitedfor all-optical switching and optical signal processing. For certainapplications, nonlinear absorption processes are of particularimportance, in which case, optimization of Im[χ⁽³⁾ _(ijkl)] ispreferred.

[0097] Nonlinear Optical Properties of Quantum Dots

[0098] In general, three mechanisms are principally responsible for χ⁽³⁾nonlinearities in quantum dots. These effects fall into the broadcategories of resonant, nonresonant, and near-resonant effects. Thesecategories can be further subdivided into degenerate (e.g., all lightbeams have the same wavelength) and non-degenerate (e.g., one or morelight beams have different wavelengths) cases.

[0099] 1) Resonant Effects:

[0100] Resonant processes typically result from a change in electronicproperties upon resonant excitation (e.g., the linear absorption oflight). This leads to a corresponding change in refractive index,following the Kramers-Kroenig relations. The magnitude of an absorptionchange, and hence the optical nonlinearity, is directly related to theground state absorption cross-section modified by any excited stateabsorption. In the case of a material with discrete states, such asmolecules or quantum dots, the optical nonlinearity results fromstate-filling and is related to (σ_(g)-σ_(e)) where σ_(g), and σ_(e),are the absorption cross sections of the material in the ground andexcited states respectively, with a reduction in refractive indexoccurring for a reduction in absorption. For quantum dots, furtherenhancement of χ⁽³⁾ results from unique physical phenomena such asquantum confinement, local electric field effects, and quantuminterference effects.

[0101] As indicated above, the optical nonlinearity is related to(σ_(g)-σ_(e)), so that increasing the oscillator strength of opticaltransitions from the ground state generally increases the opticalnonlinearity. In the case of quantum dots, a decrease in size increasesthe spatial overlap of the electron and hole wave functions, which inturn increases the oscillator strength. Resonant nonlinearity thereforetends to increase with decreasing size. This enhancement, however, canbe limited by any size dispersion.

[0102] Another important effect arises from the presence of one or moredefects in a quantum dot. Defects can be present as trap states withinthe quantum dot. Due to the enormous surface to volume ratio in the sizerange of quantum dots, most relevant traps exist on the surface. If notpassivated correctly, resonant excitation of a quantum dot createselectron-hole pairs that quickly relax into these surface-states. Holes,with their relatively large effective mass, tend to trap more easily,while the electrons, with their smaller effective mass, remain largelydelocalized. The result is a spatial separation of the electron and holewavefunctions and a decrease in oscillator strength, reducing themagnitude of the resulting nonlinearity. Furthermore, by tailoring therate of relaxation between the delocalized quantum dot states and thelocalized surface states, it is possible to control the response time ofthe resonant optical nonlinearity.

[0103] Resonant nonlinearities can be utilized in both the degenerateand non-degenerate cases with respect to the wavelength of control anddata beams. In the degenerate case, the wavelength range-of-interestlies near the absorption edge. For a single beam, the absorption can besaturated, leading to an intensity dependent absorption, commonly knownas saturable absorption. For degenerate control and data beams, thecontrol beam can modulate the transmission of the data beam, leading toan optical modulator. The refractive index change caused by theabsorption change can also be utilized. Due to the broad electronicabsorption in semiconductors in general and quantum dots in particular,resonant nonlinearities can be observed for the case where the controlbeam and the data beam are non-degenerate. In this case, the controlbeam can be of higher photon energy, such that carriers are generatedwhich relax (primarily via phonon emission) towards the band edge, wherethe absorption bleaching and/or excited state absorption can affect thedata beam of lower photon energy (but still resonant).

[0104] 2) Nonresonant Effects:

[0105] In contrast to resonant nonlinearities, where linear absorptionof light is typically required, non-resonant nonlinearities typically donot require single-photon absorption of light. As a result, nonresonantnonlinearities are intrinsically fast since excited state relaxation isnot required. However, nonresonant nonlinearities are generally smallerthan resonant nonlinearities, due to the lack of strong single-photonresonance enhancement (although multi-photon resonance can be utilizedto enhance the nonresonant nonlinearity).

[0106] There are three primary enhancement factors that can be utilizedfor nonresonant nonlinearities in quantum dots: quantum confinement,multi-photon resonance enhancement, and local-field effects. Quantumconfinement provides an increase in oscillator strength due to enhancedwavefunction overlap (as described above), which enhances χ⁽³⁾.Multi-photon resonances can be utilized in the absence of single-photonresonances to enhance the nonresonant nonlinearity. However,multi-photon resonances can introduce unwanted nonlinear absorptivelosses. For certain applications, the ideal situation is one where therelevant light beams are just below the threshold of a multi-photonresonance, thereby allowing some resonant enhancement withoutsignificant nonlinear absorption loss. Finally, local field effects canbe utilized to enhance the nonresonant χ⁽³⁾. In particular, for ananocomposite material in which quantum dots with dielectric constant ε₁are imbedded in a matrix material with dielectric constant ε₂, anexternally applied electric field (such as that originating from anelectromagnetic light source) can be locally enhanced at the quantumdots if ε₁>ε₂, with the magnitude of the enhancement related toΔε=ε₁−ε₂. Such a situation can arise by embedding the quantum dots in alower index matrix material. When illuminated by light, the electricfield at the quantum dots is enhanced compared to the incident externalfield, in turn leading to an increase in the overall nonlinear response.This enhancement increases with size of the quantum dots as the quantumdot bandgap energy decreases, resulting in an increase in dielectricconstant (ε₁).

[0107] Nonresonant nonlinearities can be utilized in the non-degeneratecase as well. In this case, the control beam can have either higher orlower photon energy than the data beam. One advantage of thenon-degenerate case is that enhancement of cross-phase modulation (thecontrol beam inducing an index change seen by the data beam) can occurwithout enhancement of self-phase modulation (the data beam affectingitself by the self-induced index change), which can cause somedeleterious effects for telecommunications data streams.

[0108] 3) Near-Resonant Effects:

[0109] Near-resonant nonlinearities can be classified into twocategories: degenerate (typically close to resonance) or non-degenerate(typically with one beam resonant and the other beam nonresonant). Inthe former case, the beams are typically very close to the resonanceedge, i.e., just above, just below, or exactly at the edge of resonance,so that either no direct excitation of the material occurs throughlinear absorption or very little direct absorption occurs. Thenon-degenerate case is perhaps the more useful situation, as therefractive index change induced by resonant excitation via a controlbeam causes a phase change for the data beam that is below resonance (soas to minimize losses due to single- or multi-photon absorption). Forexample, the refractive index change due to the absorption saturationthat extends to photon energies well below the absorption edge can beutilized, where carriers can be directly generated using the controlbeam instead of generating carriers via two-photon absorption using ahigh-intensity data beam. In addition, the excitation of free carriersin quantum dots due to absorption of control beam photons can lead to arefractive index change caused by other free carrier effects. Forexample, due to their small size, quantum dots typically intrinsicallyhave high free carrier densities for even single photon absorption(e.g., ˜10 ¹⁸ carriers/cm³ for one photon absorption in a single quantumdot). This leads to effects such as quantized Auger recombination andenhanced reflectivity (due to a large plasma frequency) at high enoughcarrier densities (e.g., ˜10 ²⁰ carriers/cm³).

[0110] Size Dependence

[0111] From the discussion above, the size dependence (for a givenquantum dot material) of both resonant and nonresonant nonlinearprocesses can be derived. Typically, for resonant optical nonlinearity,the magnitude of the nonlinearity increases as the quantum dot sizedecreases, decreases as the number of quantum dots with traps thatlocalize electrons or holes increases, and decreases as the sizedispersion increases.

[0112] Typically, for nonresonant processes, the optical nonlinearityincreases with increasing quantum dot size, increases with increasingindex of refraction of the quantum dot, increases with decreasing indexof refraction of the surrounding matrix material. There is the caveatthat these trends may not continue indefinitely to all sizes of quantumdots but can be useful as aids in practical design considerations. Bycarefully tailoring the specific size of the quantum dot, resonanteffects, nonresonant effects, or both, can be used to optimize theresulting nonlinear response.

[0113] Quantum Dot Material Dependence

[0114] One important consideration for a material forming a quantum dotis that, for bound electrons, the optical nonresonant nonlinearitytypically depends on the energy gap of the material as 1/E_(g) ^(n),where n typically ranges from about 4 to about 6. The nonresonantnonlinearity therefore can increase significantly as the energy gapdecreases. This trend favors a combination of large quantum dot sizesand materials with intrinsically small bandgap energies. At the sametime, however, the photon energy in the wavelength range-of-interest canaffect the choice of material and quantum dot size in order to avoidsignificant linear and nonlinear absorption. Specifically, the materialin the bulk form desirably should have an energy gap roughly equal to orgreater than the photon energy in the wavelength range-of-interest for adata beam in order to exploit quantum confinement effects that shift theenergy gap to higher energies. At the same time, to avoid significantmulti-photon absorption effects, the energy gap of the materialdesirably should be sufficiently large that the energy gap of theresulting quantum dot is greater than two times the photon energy of thedata beam photons.

[0115] For the case of nonresonant optical nonlinearities, these twoconcerns specify opposing trends that bracket the energy gap of thematerial of choice for quantum dots according to some embodiments of thepresent invention. The material in the bulk form desirably should havean energy gap less than this bracketed energy in order to exploitquantum confinement effects that shift the energy gap to higherenergies. As an example, to avoid two-photon losses in degenerateall-optical switching components operating near 1550 nm (correspondingto a photon energy of 0.8 eV) and to also take advantage of the 1/E_(g)^(n) behavior of the nonlinear response, the quantum dot energy gapshould be less than but close to 775 nm (or greater than but close to1.6 eV).

[0116] Enhanced Optical Properties

[0117] In addition to size-dependent spectral characteristics, quantumconfinement can also result in an enhancement in the magnitude ofvarious optical and electronic properties due to a redistribution of thedensity of states. Properties such as absorption cross-section andexcited-state polarizability have been found to be enhanced by severalorders of magnitude over bulk materials. χ⁽³⁾ can also be enhanced byquantum confinement, as described previously.

[0118] Additional Effects

[0119] There are many practical definitions of a figure-of-merit (FOM)that take into account the many parameters that can be important andrelevant for all-optical switching. One example of such a FOM is definedas $\frac{\Delta \quad n}{\alpha \cdot \tau},$

[0120] where Δn is the induced refractive index change, α is the linearand nonlinear absorption coefficient, and τ is the response time of thematerial. For this FOM, which is particularly relevant for resonantoptical nonlinearities where light absorption is used, the larger theFOM, the better will be the performance of the all-optical switching. Adefinition of a FOM useful for nonresonant optical nonlinearities, whereideally no or little light absorption occurs, is 2γ/βλ, where γ is thenonlinear index of refraction, β is the two-photon absorptioncoefficient, and λ is the wavelength of operation. In this case, usefulall-optical switching typically occurs when FOM>1. According to someembodiments of the invention, the following effects can be important forthe formation of nanocomposite materials with a FOM in a usable rangefor practical optical switching.

[0121] The Effect of Defects on FOM:

[0122] Defects within quantum dot materials can have a substantialnegative impact on their performance as nonlinear optical materials.Defects in the core and/or surface of the quantum dot can yield directabsorption of below-bandgap photons, increasing optical losses, anddecreasing the overall FOM. As a result, while χ⁽³⁾ may be high, thematerial can still be inappropriate for optical switching. The effect ofdefects on optical switching using quantum dots has not been previouslyconsidered as discussed herein.

[0123] One important aspect of some embodiments of the invention isthat, for quantum dots to be used as a nonlinear optical material, theydesirably should comprise a substantially defect-free core. In thiscase, the term “defect” typically refers to defects with energy belowthe energy gap of the quantum dot core or within the energy range of thewavelength range-of-interest. Additionally, the surface of quantum dotsshould be well passivated, such that there are substantially no defectstates. Passivation can be accomplished, for example, through theinclusion of appropriate surface ligands in the ligand layer to bind todefect sites and remove them from the energy gap. Alternatively, or inconjunction, passivation can be achieved by applying a shell to thequantum dot core to fill or eliminate the defect sites. In this case,the shell material is preferably a material with an energy gap that ishigher than that corresponding to the wavelength range-of-interest, andmore preferably higher than the energy gap of the quantum dot core.Additionally, the shell desirably should be substantially defect-free orshould have defects that can be eliminated through the inclusion ofappropriate surface ligands.

[0124] Concentration Effects:

[0125] One important aspect of some embodiments of the invention is thatthe nonlinear properties of a material including quantum dots can besubstantially affected by correlated interactions between two or morequantum dots. In particular, while χ⁽³⁾ can be proportional toconcentration of quantum dots at low concentrations, as theconcentration increases, the individual quantum dots can get closeenough to interact with each other, producing collective phenomena thatcan further enhance nonlinearity. This effect is seen in FIG. 5(b),which illustrates concentration dependence of the optical nonlinearityof engineered nonlinear nanocomposite materials doped with silicon andgermanium quantum dots, according to an embodiment of the invention. Thesilicon and germanium quantum dots were made in accordance with themethods described herein. The vertical axis represents the nonlinearindex of refraction γ, and the horizontal axis represents the relativeconcentration of quantum dots in a matrix material. As shown in FIG.5(b), y can increase superlinearly with concentration at sufficientlyhigh concentrations. The effect of concentration (and particularly thesuperlinear concentration dependence) on optical switching using quantumdots has not been previously considered as discussed herein.

[0126] For FIG. 5(b), γ arises as a result of nonresonant degeneratenonlinearities. The values attained for γ are particularly large. Asshown in FIG. 5(b), the nanocomposite material doped with siliconquantum dots has γ as high as about 8×10⁻⁵ cm²/W, which is 9 orders ofmagnitude larger than the bulk material from which the silicon quantumdots are fabricated (bulk silicon has a nonresonant degenerate γ ofabout 8×10⁻¹⁴ cm²/W). Additional nonlinear enhancement can be inducedthrough the appropriate selection of molecular species in the ligandlayer (see discussion below on Molecular Tethers).

[0127] Summary of Nonlinear Optical Properties of Quantum Dots

[0128] Enhancement and tunability of the optical nonlinearity inindividual quantum dots and multi-quantum dot nanocomposites, combinedwith substantially defect free and/or well passivated quantum dot cores,provide the engineered nonlinear nanocomposite materials according tosome embodiments of the current invention. Such nanocomposite materialscan satisfy various characteristics for an ideal χ⁽³⁾ based opticalmaterial that include (but are not limited to): large Re[χ⁽³⁾ _(ijkl)]in the wavelength range-of-interest; a multi-photon transition that canbe tuned to maximize near-resonance enhancement while minimizing opticalloss due to absorption; the use of non-degenerate control and data beamswhere the control beam is resonant and induces a large index change atthe data beam wavelength while introducing low optical loss at thatwavelength; the use of degenerate control and data beams to allowcascading of devices; and low optical loss due to absorption by defects.

[0129] Colloidal Quantum Dots

[0130] Structures comprising quantum dots can be fabricated using vapordeposition, ion-implantation, photolithography, spatially modulatedelectric fields, semiconductor doped glasses, strain-induced potentialvariations in quantum wells, atomic width fluctuations in quantum wells,and a variety of other techniques. Preferably, quantum dots are formedor used in a form that can be easily incorporated into flexible orengineered optical materials or devices. In addition, it is desirable toseparate the optical properties of the quantum dots from those of amatrix material to achieve a sufficiently large FOM with reducedabsorption and/or scattering by the matrix material.

[0131] In a preferred embodiment, the current invention comprisescolloidal quantum dots. Colloidal quantum dots are freestandingnanostructures that can be dispersed in a solvent and/or a matrixmaterial. Such colloidal quantum dots are a particularly preferredmaterial for some embodiments of the current invention because they canbe more easily purified, manipulated, and incorporated into a matrixmaterial.

[0132] It will be apparent to one of ordinary skill in the art that thedefining characteristic for a “colloidal” quantum dot is that it is afreestanding nanostructure. The method of fabrication, size, and shapeof the particular colloidal quantum dot do not bear on itsclassification.

[0133] Chemical Properties

[0134] Chemically Controllable Surface

[0135] According to some embodiments of the invention, a unique physicalcharacteristic of quantum dots is that, while the core can comprise acrystalline semiconductor material, the surface can be coated with avariety of different organic and/or inorganic materials. These surfacecoatings (e.g., shells or ligand layers) can impart stability andchemical activity, as well as passivation of electrically and opticallyactive defect sites on the quantum dot surface. These surface coatingsare optionally substantially different in chemical nature than theinorganic core. As a result, while quantum dots can comprise primarily ahighly nonlinear semiconductor material, they substantially appear tothe surrounding material as surface ligands. As such, the processabilityand chemical stability of this highly nonlinear and tunable opticalmaterial can primarily be a function of the surface layer and not afunction of the material that provides the majority of the opticalcharacteristics.

[0136] Surface ligands are preferably bi-functional. By bi-functional,it is meant that there are at least two portions of the surface ligandsuch that one portion interacts primarily with the quantum dot surface,while the second portion interacts primarily with the surroundingenvironment (e.g., solvent and/or matrix material). These at least twoportions of the surface ligand may be the same or different, contiguousor noncontiguous, and are optionally contained within two or moredifferent molecular species that interact with each other to form theligand layer. The at least two portions can be selected from a groupconsisting of hydrophilic groups, hydrophobic groups, or amphiphilicgroups. The interaction of each of the at least two portions and thequantum dot or surrounding environment can be covalent or noncovalent,strongly interacting or weakly interacting, and can be labile ornon-labile. The at least two portions can be selected independently ortogether.

[0137] In some embodiments of the current invention, the surface ligandsare selected such that the portion that interacts with the quantum dotpassivates defects on the surface such that the surface is madesubstantially defect-free. At the same time, the portion that interactswith the environment is selected specifically to impart stability andcompatibility (e.g., chemical compatibility or affinity) of the quantumdot within a matrix material that is selected for a specificapplication. Simultaneously satisfying both of these requirements is animportant aspect of certain embodiments of the current inventionrelating to the development of an engineered nonlinear nanocompositematerial. Alternative methods of achieving these requirements include(but are not limited to): 1) Passivating the surface of the quantum dotindependent of the ligand layer (e.g., using a shell or creating anintrinsically defect free surface), while the environmentalcompatibility is imparted by the surface ligands, or 2) imparting bothpassivation and environmental compatibility independent of the ligandlayer. Achieving passivation of the surface of quantum dots is oneadvantage of using colloidal quantum dots over alternate approaches.

[0138] Through the appropriate selection of surface ligands, quantumdots can be incorporated into a variety of matrix materials such as, forexample, liquids, glasses, polymers, crystalline solids, and evenclose-packed ordered or disordered quantum dot arrays. The resultingnanocomposite materials can be formed into homogeneous, high-qualityoptical films of quantum dots. Alternatively, the chemistry can beselected to allow dispersion of the quantum dots into a matrix materialwith a controllable degree of aggregation, forming micron or sub-micronsized clusters. The result is an increased local fill-factor and anenhanced local field effect that may further increase the nonlinearresponse of the nanocomposite materials of embodiments of the presentinvention.

[0139] An important aspect of some embodiments of this invention relatesto effectively separating the optical properties of the quantum dotsfrom the optical, chemical, mechanical, and other properties of thematrix material. In this aspect, it is possible to combine the largenonlinearities of quantum dots with the ease of handling andprocessability of a matrix material such as a standard polymer. Thus,this aspect provides two additional features of an ideal χ⁽³⁾ basedoptical material: physical and chemical compatibility with specificdevice architectures and the ability to be easily processed forincorporation.

[0140] Molecular Tethers:

[0141] In addition to conveying stability and chemical compatibilitywith the surrounding environment, the ligand layer can optionally beused to tailor the physical, optical, chemical, and other properties ofthe quantum dots themselves. In this case, it is not just the chemicalnature of the surface ligand but also the interaction of the surfaceligand with the quantum dot that imparts an additional level of controlover the physical, optical, chemical, and other properties of theresulting nanocomposite material. We refer herein to any molecule,molecular group, or functional group coupled (e.g., chemically attached)to the surface of a quantum dot that imparts additional functionality tothe quantum dot as a “molecular tether”. In some cases, the moleculartether can be electrically active, optically active, physically active,chemically active, or a combination thereof. The inclusion of moleculartethers into a quantum dot structure is an important aspect of someembodiments of the present invention.

[0142] Active species are used to precisely control the electrical,optical, transport, chemical, and physical interactions between quantumdots and the surrounding matrix material and/or the properties ofindividual quantum dots. For instance, a conjugated bond covalentlybound to the surface of one or more quantum dots may facilitate chargetransfer out of one quantum dot and into another. Similarly, aphysically rigid active group bound in a geometry substantially normalto the surface of a quantum dot can act as a physical spacer, preciselycontrolling minimum interparticle spacing within an engineered nonlinearnanocomposite material.

[0143] As described above, collective phenomena (e.g., at highconcentrations) are an important aspect of some embodiments of thecurrent invention. This aspect can be further enhanced by allowingindividual quantum dots to interact with one another using moleculartethers that foster interactions between quantum dots. At sufficientlyhigh number densities, the molecular tethers begin to make contact withmolecular tethers from other quantum dots or with other quantum dotsdirectly. This can serve to augment nonlinearity by controlling theinteraction between quantum dots and thus increasing the degree ofcollective phenomena compared to single particle phenomena. Moleculartethers may include, but are not limited, to conducting polymers, chargetransfer species, conjugated polymers, aromatic compounds, or moleculeswith donor-acceptor pairs. These molecular tethers can foster electrondelocalization or transport and thus can increase the interactionbetween quantum dots. Additionally, the molecular tethers can beselected to facilitate high quantum dot number densities without thedetrimental aggregation that often plagues high concentration systems.

[0144] Molecular tethers can also be selected to impart stability ofquantum dots under a variety of environmental conditions includingambient conditions. Molecular tethers can optionally contain chemicallyactive groups to allow quantum dots to be attached to polymer backbones,along with other active molecules. This provides a method forcontrolling the density of quantum dots within close proximity ofmolecules that influence a variety of functions such as carriertransport or delocalization.

[0145] An additional aspect of the present invention is the use ofmolecular tethers to physically connect two or more quantum dots in a 1dimensional, 2 dimensional, or 3 dimensional structure or array. Suchquantum dot superstructures can be created to initiate multiple dotquantum interference interactions or collective phenomena yielding newand useful properties such as enhanced, nonsaturating opticalnonlinearities. The length and properties of these molecular tethers canbe tailored to enhance or generate specific quantum phenomena. Thesenanostructures can have the properties of single quantum dots or anensemble of quantum dots depending on the nature of the moleculartethers. For certain applications, more than one type of moleculartether can be used to connect quantum dots.

[0146] The quantum dots according to some embodiments of the inventionexemplify microscopic conditions that enhance the nonresonant opticalnonlinearity arising from local electric field effects described above.Whether the quantum dot surface is terminated with oxide or ligand layer(e.g., molecular tethers), the result is a particle (e.g., a core of thequantum dot) with dielectric constant ε₁ surrounded by an environment(e.g., the surface oxide layer or molecular tethers) with dielectricconstant ε₂ where ε₁>ε₂. Therefore, the enhancement of the nonresonantoptical nonlinearity can be engineered by the judicious choice of oxideor molecular tether without resorting to a surrounding bulk matrixmaterial. In other words, a single quantum dot as described in thispatent should exhibit an enhanced nonresonant optical nonlinearity sincethe surface layer functions as a surrounding matrix material with alower dielectric constant. Optionally, molecular tethers can be used toconnect quantum dots together without a separate matrix material. Inthis case, an extrinsic matrix material is not required since theindividual interconnected quantum dots exhibit an enhanced localelectric field effect.

[0147] A preferred approach of attaching appropriate molecular tethersto a quantum dot surface can be thought of as essentially treating aquantum dot as a very large molecule (e.g., a macro-molecule) and themolecular tethers as functionalizations of this large molecule. Thiscreates a large three-dimensional structure with enhanced nonlinearoptical properties resulting from the combination of quantum effectsfrom the quantum dot and carrier polarization and delocalization effectsfrom the molecular tethers and from the interaction of these twoeffects. These properties can be tailored by the choice of moleculartethers. In addition, a quantum dot can also represent a large andstable reservoir of polarizable charge that also contributes to a largenonlinear optical response.

[0148] Macroscopic Quantum Dot Solids

[0149] Macroscopic solids can be fabricated in which quantum dots form asubstantially close-packed array (e.g., a cubic closed-packed array) inthe absence of an extrinsic matrix material. These “quantum dot solids”can either be crystalline, polycrystalline, or amorphous. Whilecontaining a relatively high density of quantum dots, quantum dot solidscan still be easily processed since, during formation, the quantum dotscan be dispersed in a solvent that is subsequently removed. Uniformsolid quantum dot films, for instance, can be formed using standardspin-coating techniques as, for example, described in C. R. Kagan etal., “Long-range resonance transfer of electronic excitations inclose-packed CdSe quantum-dot solids,” Phys. Rev. B 54, 8633 (1996), thedisclosure of which is incorporated herein by reference in its entirety.In addition, surface ligands can still be selected to impart solventcompatibility and appropriate chemical stability to the final quantumdot solid. In contrast to the interconnected material described above,these macroscopic quantum dot solids are typically not held together bymolecular bonds but rather by Van der Waals forces.

[0150] High-quality optical materials can be fabricated from quantum dotsolids with substantially homogeneous optical properties throughout thematerial. The density of quantum dots can be tuned by modifying thelength and/or structure of the surface ligands. Careful selection ofsurface ligands can produce continuously tunable densities up to amaximum fill-factor of about 75% by volume of the quantum dot solid,preferably between about 0.005% and 75% by volume (e.g., between about10% and 75% by volume, between about 30% and 75% by volume, betweenabout 50% and 75% by volume, or between about 60% and 75% by volume).The surface ligands are optionally removed partially or completely byheating or chemical treatment after the quantum dot solid is formed.More specifically, the length of the surface ligands can be used todefine the spacing between quantum dots. By combining the ability tocreate density-controlled quantum dot solids with variable densityquantum dots in a matrix material, the concentration of quantum dots,and therefore the nonlinear index of refraction of the materialsdescribed herein, can be tuned over many orders of magnitude.

[0151] In the case of quantum dot solids, the surface ligands can takethe place of an extrinsic matrix material according to some embodimentsof the current invention. In the case of close-packed quantum dots inwhich the surface ligands have typically been removed, the quantum dotsthemselves are considered to form their own “intrinsic” matrix material.Quantum dot solids according to some embodiments of the invention can befabricated in a variety of ways, such as, for example, described in C.B. Murray et al., “Self-Organization of CdSe Nanocrystallites intoThree-Dimensional Quantum Dot Superlattices,” Science 270, 1335 (1995),C. R. Kagan et al., “Long-range resonance transfer of electronicexcitations in close-packed CdSe quantum-dot solids,” Phys. Rev. B 54,8633 (1996), and U.S. Pat. No. 6,139,626 to Norris et al., entitled“Three-dimensionally patterned materials and methods for manufacturingsame using nanocrystals” and issued on Oct. 31, 2000, the disclosures ofwhich are incorporated herein by reference in their entirety. Quantumdot solids according to some embodiments of the invention can befabricated with a variety of different quantum dot materials, sizes, andsize distributions. It is also possible to form mixed quantum dot solidscomprising a plurality of quantum dot materials, sizes, and sizedistributions.

[0152] Engineerable Nonlinear Nanocomposite Materials

[0153] One embodiment of the present invention comprises an engineerednonlinear nanocomposite material that combines the large nonlinear andsize dependent optical properties of quantum dots with theprocessability and chemical stability of a matrix material and/or achemically controlled quantum dot surface. By separately selecting thesize and material of the quantum dot, the surface ligands, the matrixmaterial, and the density of quantum dots within the matrix material,one can independently tune various significant characteristics indesigning an ideal nonlinear optical material.

[0154] In particular, this embodiment of the present invention comprisesthe following characteristics that, taken together, in part or in whole,provides a substantially improved nonlinear optical material over whatis known in the art.

[0155] The effects of quantum confinement and the specific selection ofquantum dot material is used to create extremely large opticalnonlinearities, specifically Re[χ⁽³⁾ _(ijkl)], in the data beamwavelength range-of-interest, while the energies of single- andmulti-photon absorption features are selected to minimize absorptiveloss of the data beam and heating and to optimize resonant enhancementeffects. This optimization can include the use of appropriately chosennon-degenerate control and data beams. Alternatively, the nonlinearabsorption mechanisms can be enhanced, e.g., Im[χ⁽³⁾ _(ijkl)] can beoptimized, depending upon the application.

[0156] The matrix material is selected, independent of the quantum dotmaterial and size, with the desired chemical and mechanical propertiesto impart physical and chemical compatibility with the specific devicearchitecture and materials as well as the process of incorporation intodevices.

[0157] The surface ligands of the quantum dots are selected tofacilitate homogeneous incorporation of the quantum dots into theselected matrix material and are optionally selected to facilitatecontrolled aggregation of quantum dots within the selected matrixmaterial.

[0158] The density of quantum dots in the matrix material is selected toprecisely tune the linear index of refraction to match the boundaryconditions for a given device architecture (in the case of high-indexmaterials, a quantum dot solid can be used).

EXAMPLE 1

[0159] This example describes a preferred embodiment in which anengineered nonlinear nanocomposite material is incorporated into anonlinear directional coupler that utilizes nonresonant or near-resonantnonlinearities. In the current example, the waveguide core is fabricatedfrom doped silica with an index of refraction of 1.52 at 1.55 μm. Itwill be recognized by one of ordinary skill in the art that doped silicacan have an index of refraction over a wide range of values. The currentexample is not meant to limit the scope of the invention, and it will beunderstood that variations on this example can extend to waveguide coreswith an arbitrary index of refraction.

[0160] In the case of a nonlinear directional coupler, light isevanescently coupled between two waveguide cores such that a signalentering one waveguide core oscillates between the two as a function ofthe interaction length. By choosing an appropriate length, the light canbe coupled completely into one or the other of the two waveguide cores(i.e., the “off” state can be transmission through one or the otherwaveguide core by appropriate device design). By changing the index ofrefraction between the waveguide cores, it is possible to switch theoutput waveguide core from the “off” state to the other waveguide core(i.e., the “on” state) for a fixed length device. An index change from aχ⁽³⁾ based nonlinear material can yield extremely fast opticalswitching. However, so far no single material has been appropriate for acommercial optical switch based on a nonlinear direction coupler.

[0161] The active material in this optical device desirably should havea large nonlinear response in the data beam wavelengthrange-of-interest. It is also desirable (primarily for nonresonantnonlinearities) to maximize resonant enhancement, while simultaneouslyavoiding significant single-or multi-photon absorption. At the sametime, the linear index of refraction of the active material desirablyshould be less than that of the core material and be close to that ofthe rest of the cladding to avoid disruption of the optical mode aslight is guided into the active region.

[0162] In this example, depicted in FIGS. 6(a) through 6(e), the devicecomprises doped silica waveguide cores 602 and 604 (n=1.552) fabricatedon a doped silica substrate 606 (n=1.515). As shown in FIG. 6(b), theother three sides of the waveguide cores 602 and 604 are initiallysurrounded by air (n=1), as is the space between the waveguide cores 602and 604 in the interaction region. The space around the waveguide cores602 and 604 is then filled with an engineered nonlinear nanocompositematerial 608 (n=1.515) to match the waveguide boundary conditions of thesubstrate 606, as shown in FIG. 6(c). By illuminating the interactionregion with trigger-pulses as shown in FIGS. 6(d) and 6(e), the index ofrefraction between the waveguide cores 602 and 604 is changed,activating the switch.

[0163] Operation of this switch is slightly different than what iscommonly described in the art. It is best understood by presupposingthat the directional coupler length is chosen such that in theinactivated state, the two waveguide cores 602 and 604 exchange energysuch that each output will receive substantially half of the power fromeach input (acting as a 3 dB coupler). If the illumination is such thatthe index of refraction increases in the nonlinear nanocompositematerial 608, the interaction between the two propagating waveguidecores 602 and 604 will decrease, leading to a reduction in the dataenergy transferred between the cores 602 and 604, forcing the switchcloser to a bar state. If the illumination is such that the index ofrefraction decreases in the nonlinear nanocomposite material 608, theinteraction between the two cores 602 and 604 increases, increasing theenergy transferred between the cores 602 and 604, forcing the switchcloser to a cross state. One skilled in the art will recognize that thistransfer function is cyclic and that further reduction of the index ofrefraction of the nonlinear nanocomposite material 608 will result inoscillations between the cross and bar states. If desired, the length ofnonlinear directional coupler may be chosen to include severaloscillations in the inactive state, leading to an effective bias in thetotal oscillations.

[0164] The engineered nonlinear nanocomposite material 608 for thisexample comprises silicon oxide coated silicon quantum dots ororganic-terminated silicon or germanium quantum dots dispersed in apoly(methyl methacrylate) polymer matrix material (PMMA; n=1.49). PMMAis chosen here due to its desirable optical properties for use in the1.55 μm range and its ease of processing in waveguide structures.Examples of these desirable optical properties include high opticaltransmissivity in the visible wavelength, relatively low absorption near1550 nm, and low birefringence (as low as 0.0002 at 1550 nm has beenobserved).

[0165] In order to optimize degenerate nonresonant switching at 1.55 μm,silicon quantum dots with a diameter of around 4 nm are used, placingthe 2-photon absorption peak at higher energy than the spectral energyrange-of-interest. This is sufficient to minimize 2-photon absorptionthat may result in signal loss and heating, while maintaining asignificant resonance enhancement at the wavelength of the triggerpulse. This particular combination of quantum dot material and size alsoyields a maximum in χ⁽³⁾ at 1.55 μm. To maximize near-resonantswitching, the appropriate choice of control wavelength and quantum dotresonance at the control wavelength desirably should be chosen thatminimizes or reduces absorption loss at the data wavelength.

[0166] To facilitate incorporation of the quantum dots into PMMA, thesilicon or germanium quantum dots can be coated with a ligand layercomprising a long-chained hydrocarbon with a methacrylate functionalgroup at the end. Alternatively, any functional group compatible withPMMA can be used. Quantum dots and PMMA are dissolved in an organicsolvent, such as toluene, and applied to the device as shown in FIG.6(c). The concentration of PMMA is determined based on the desiredthickness of the final nanocomposite material and the method ofapplication. In the case of spin-coating, a 5% PMMA solution isappropriate. The concentration of quantum dots is selected such that thefinal nanocomposite material, after deposition, has a linear index ofrefraction of 1.515. This is determined by calibrating the initialconcentration of quantum dots (as measured by the absorptioncharacteristics) to the final index of refraction of a PMMA-quantum dotfilm deposited in the method to be used. The linear index of the filmcan be measured using ellipsometry or the like.

[0167] After spin-coating the polymer-quantum dot solution over thedevice, the solvent is allowed to evaporate, leaving an engineerednonlinear nanocomposite coated device as shown in FIG. 6(c). The indexof refraction around all sides of the waveguide cores 602 and 604 ismatched and optimized for the specific device. At the same time, χ⁽³⁾and the resonance conditions for 1.55 μm are independently tuned foroptimum switching performance. As a final aspect of the current example,based on the known intensity of the trigger-pulse and the resultingnonlinear response of the engineered nonlinear nanocomposite material608, the active length of the device is selected to provide optimalswitching performance. This can be done by limiting the illuminationarea of the trigger-pulse to define the active area as in FIG. 6(d) orby designing the specific waveguide structure with the appropriateinteraction length as in FIG. 6(e). The actual active length can bedetermined empirically or through simulation.

[0168] By increasing the index of refraction of waveguide cores,substantially larger concentrations of quantum dots can be incorporatedinto the active material while retaining functionality of the switch.This can yield substantially higher switching efficiency. For example,as shown in FIG. 6(f), with silicon waveguide cores 610 and 612 havingan index of refraction of ˜3.4, an active material 614 desirably shouldhave an index of refraction equal to or less than 3.39 to achieveefficient waveguiding through the active region. This allows densitiesof quantum dots as high as those of close-packed quantum dot solids(either crystalline or amorphous).

EXAMPLE 2

[0169] To highlight the flexibility of embodiments of the currentinvention, this example describes a second preferred embodiment in whichan engineered nonlinear nanocomposite material may be used in awaveguide nonlinear Mach-Zehnder (MZ) interferometer. In this case, asshown in FIGS. 7(a) through 7(f), a waveguide core is fabricated frompartially oxidized silicon with an index of refraction of 2.4 at 1.55μm. Once again, it will be apparent to one of ordinary skill in the artthat partially oxidized silicon can have a range of indices ofrefraction, and that 2.4 is not meant to limit the scope of theinvention. Variations on this example comprising other possible indicescan be used depending on the specific application.

[0170] In the nonlinear MZI of the present example, a data signaltraveling along a waveguide core is split into two separate anduncoupled waveguide arms with a defined phase relation between them. Thesignals travel along the arms for a predetermined length and are thenrecombined. Phase differences resulting from the propagation of thelight in each arm result in constructive or destructive interference ofthe signals in the output waveguide core. By modulating the index ofrefraction of one or both of the arms, the output signal can be switchedon or off by creating a relative 0- to π-phase shift between thesignals. One of ordinary skill in the art will realize that furtherchanges in index of refraction will result in cyclic exchange betweenthe on and off states. An index change from a χ⁽³⁾ based nonlinearmaterial would yield extremely fast optical switching; however, so farno single material has been appropriate for a commercial switch based onthis device.

[0171] As with the example above, the active material in this devicedesirably should have a high nonlinear response in the wavelengthrange-of-interest and no significant absorption. In this case, however,the nonlinear material is incorporated directly into the waveguide core.As such, the index of refraction of the engineered nonlinearnanocomposite desirably should be greater than that of the claddingmaterial and be close to that of the core to avoid disruption of theoptical mode as light moves into the active region.

[0172] In this example, as shown in FIG. 7(a), the device comprises apartially oxidized waveguide core (n=2.4) fabricated on a silicasubstrate (n=1.45) and surrounded by a silica cladding on three sides.The top of the waveguide core is bounded by air (n=1). A section of oneof the waveguide arms is etched away as shown in FIG. 7(b), filled withan engineered nonlinear nanocomposite material (n=2.4) as shown in FIG.7(c) to match the boundary conditions of the waveguide core, and thenpolished as shown in FIG. 7(d). By illuminating the active region withtrigger-pulses as shown in FIGS. 7(e) and 7(f), the index of refractionin one arm is changed, thus activating the switch. A preferredengineered nonlinear nanocomposite material for this example comprisessilicon oxide coated silicon quantum dots formed into a close-packedquantum dot solid with index of refraction tuned to 2.4.

[0173] In order to optimize switching at 1.55 μm, silicon quantum dotswith a diameter of 4 nm are used, placing the 2-photon absorption peakat higher energy than the spectral energy range-of-interest. This issufficient to eliminate or reduce 2-photon absorption that may result insignal loss and potential heating of the device by the trigger-pulse.This particular combination of material and size also yields a maximumin χ⁽³⁾ at 1.55 μm. To maximize near-resonant switching, the appropriatechoice of control wavelength and quantum dot resonance at the controlwavelength desirably should be chosen that minimizes or reduces anyabsorption loss at the data wavelength.

[0174] In order to achieve precise index of refraction control withinthe waveguide arm, surface ligands desirably should be selected to yielda specific particle-to-particle spacing within the final quantum dotsolid. This can be achieved by measuring the index of refraction of manythin-films, formed by the method to be used, with quantum dotscomprising different types of surface ligands. By using ellipsometry orthe like, the index of refraction resulting from each type of surfaceligand and deposition method can be determined and calibrated fordetermining the optimum conditions for the final device deposition. Inthe case of the present example, an index of 2.4 corresponds roughly toa packing density of 70% by volume. A short-chained hydrocarbon ispreferable in this case, such as a butyl- or other alkyl group.

[0175] The quantum dots, in a solvent of hexane or toluene, arespin-coated over the surface of the device, filling the open region ofthe waveguide arm as shown in FIG. 7(c). A slow spin speed ispreferable, since the thickness of the material in the waveguide arm canbe controlled by polishing the overflow off the surface (1000 rpm). Theconcentration of quantum dots in the solution should be high, preferablyin the range of 1 nM to 1M, more preferably 10 μM to 1 mM.

[0176] After spin-coating, the solvent is allowed to evaporate, creatinga close-packed quantum dot solid filling the open region of thewaveguide arm as shown in FIG. 7(c). The surface is then polished toprovide an optical-quality interface on the topside of the device in theactive region as shown in FIG. 7(d). The index of refraction of theengineered nonlinear nanocomposite is matched to that of the waveguidecore of the arm and optimized for the specific device. At the same time,χ⁽³⁾ and the resonance conditions for 1.55 μm are independently tunedfor optimum switching performance. As a final aspect of the currentembodiment, based on the known intensity of the trigger-pulse and theresulting nonlinear response of the engineered nonlinear nanocompositematerial, the active length is selected to provide optimal switching.This can be done by designing the etched length of the waveguide arm tothe desired active length as in FIG. 7(e) or by limiting theillumination area of the trigger-pulse as in FIG. 7(f). The specificactive length can be determined empirically or through simulation.

[0177] Alternatively, a nonlinear MZ interferometer can be fabricatedwithout etching a portion of a waveguide core as shown in FIGS. 8(a)through 8(d). In this case, a engineered nanocomposite material can besimply cast on top of the entire device as shown in FIG. 8(b) with anyexcess removed as shown in FIG. 8(c), such that the active material isin evanescent contact with the signal passing through each of the arms(as well as elsewhere). By illuminating a portion of one or both arms,the active region can be defined as shown in FIG. 8(d). In thispreferred embodiment, the engineered nonlinear nanocomposite desirablyshould be designed to have an index of refraction that is compatiblewith waveguiding in the partially oxidized silicon core (e.g., n<2.4).Again, this nanocomposite material is preferably a close-packed quantumdot solid.

[0178] Had further chemical processing steps been required in either ofthe above examples, it would also be possible to select the matrixmaterial and/or surface ligands to impart stability of the engineerednonlinear nanocomposite under the required conditions.

[0179] The current embodiments not only provide a nonlinear materialwith a dramatically increased nonlinear response for use in theseoptical devices, they simultaneously provide materials that have beenengineered to have optimum linear index of refraction, 2-photonabsorption, near-resonance enhancement, and processability for eachapplication. This level of independent control of optical, chemical, andmechanical properties does not exist in other materials.

[0180] Preferred Quantum Dot Materials

[0181] Preferred quantum dots according to some embodiments of thepresent invention comprise substantially defect free quantum dots with awell-passivated surface. Preferred quantum dots also comprise a bandgapenergy that is preferably greater than the photon energyrange-of-interest (e.g., for the data beam), and more preferably greaterthan twice the photon energy range-of-interest (primarily fornonresonant nonlinearities) for its intended applications. Whilemaintaining these requirements, the material and size of the quantumdots can be interchangeable. The specific material and size can beselected as necessary to engineer the optical characteristics for aparticular application. The following provides certain preferredcharacteristics according to some embodiments of the invention:

[0182] Core-Shell Quantum Dots:

[0183] Core-shell quantum dots are particularly preferred becausedefects can result in traps for electrons or holes at the surface of aquantum dot core. These traps can degrade the electrical and opticalproperties of the quantum dot, yielding low-energy states within thebandgap of the material. An insulating layer at the surface of thequantum dot core provides a rise in the chemical potential at theinterface, which can eliminate energy states that serve as traps.Surprisingly, these trap states can actually interfere with efficientswitching or decrease the FOM of a material by contributing to single ormulti-photon absorption. Additionally, shells act to physically protectthe core material from chemical interactions such as oxidation,reduction, or dissolution. For instance, one embodiment of the presentinvention relates to the use of a shell to stabilize intrinsicallyunstable silicon or germanium quantum dots. Optionally, the shell canprovide an appropriate chemical surface for covalent or non-covalentbinding of molecules to the quantum dot, wherein the core material mayor may not provide an appropriate surface for such binding.

[0184] Preferably, a quantum dot will be substantially defect free. Bysubstantially defect free, it is typically meant that within the quantumdot there is fewer than 1 defect per quantum dot, preferablysubstantially fewer than 1 defect per quantum dot, more preferably lessthan 1 defect per 1000 quantum dots, more preferably less than 1 defectper 10⁶ quantum dots, more preferably less than 1 defect per 10⁹ quantumdots. Typically, a smaller number of defects within a quantum dottranslates into an increased photoluminescence quantum efficiency. Forcertain embodiments of the invention, a quantum dot that issubstantially defect free will typically exhibit photoluminescence witha quantum efficiency that is greater than 6 percent, preferably greaterthan 10 percent, more preferably at least 20 percent, more preferably atleast 30 percent, more preferably at least 40 percent, and morepreferably at least 50 percent.

[0185] Preferably, the core will be substantially crystalline and besubstantially defect-free. By substantially defect free, it is typicallymeant that within the core there is fewer than 1 defect per quantum dot,preferably substantially fewer than 1 defect per quantum dot, morepreferably less than 1 defect per 1000 quantum dots, more preferablyless than 1 defect per 10⁶ quantum dots, more preferably less than 1defect per 10⁹ quantum dots.

[0186] In a similar manner, the shell and/or the interface regionpreferably will be substantially defect free, where it is typicallymeant that within the shell and/or the interface region there is fewerthan 1 defect per quantum dot, preferably substantially fewer than 1defect per quantum dot, more preferably less than 1 defect per 1000quantum dots, more preferably less than 1 defect per 10⁶ quantum dots,more preferably less than 1 defect per 10⁹ quantum dots.

[0187] Size and Size-Distribution:

[0188] Another preferred characteristic of the quantum dots of someembodiments of the present invention is such that a figure-of-merit(FOM) for all-optical switching or processing can be largely insensitiveto size dispersion, contrary to results and predictions in theliterature. FIG. 9 illustrates a figure-of-merit (FOM) for all-opticalswitching with an engineered nonlinear nanocomposite material as afunction of quantum dot size, according to an embodiment of theinvention. Here, the nanocomposite material includes germanium quantumdots made with methods described herein. The FOM in this case is definedas 2γ/βλ, which is applicable for nonresonant nonlinearities. Thecriteria for effective all-optical switching is FOM>1. FIG. 9 shows howthe FOM for all-optical switching depends on the size of the quantumdots. It can be seen that the FOM exceeds 1 for a large size dispersion,e.g., for diameters ranging from 3 nm to 6 nm. Similar results can beobtained with the other quantum dots described herein, e.g., siliconquantum dots. Therefore, some embodiments of the present invention avoidthe need for a substantially monodispersed size distribution of quantumdots while substantially improving switching characteristics andefficiency over previous uses of quantum dots as nonlinear materials.The effects of size distribution and specifically how the FOM ofswitching depends on the quantum dot size has not been previouslyconsidered in detail.

[0189] Shape and Shape Distribution

[0190] Quantum dots can be fabricated in a variety of shapes, including(but not limited to) spheroids, rods, pyramids, cubes, and othergeometric and non-geometric shapes. For shapes that are not sphericallysymmetric, a distribution of orientations can result in an effectivebroadening of the size distribution as seen by incident light. To avoidthe need for orientation of quantum dots within a matrix material, thepreferred quantum dot shape is spherical, according to some embodimentsof the invention. Spherical quantum dots are also preferred fornanocomposites comprising oriented quantum dots. Alternatively, anotherpreferred embodiment comprises spheroid or substantially sphericalquantum dots, with an aspect ratio restricted to between 1±(% sizedistribution) or with an aspect ratio between approximately 0.8 and 1.2.In this case, orientation plays an insignificant role in theinhomogeneous broadening of the spectral features. For similar reasons,the preferred quantum dot will also be substantially monodisperse inshape. These considerations regarding the importance of shape and/orshape-distribution constitute an improvement in the use of quantum dotsas a nonlinear material.

[0191] It should be recognized that an arbitrary shape may still bepreferred as long as the relative orientation dependence of thebroadening of the linear and nonlinear optical properties is less thanthe broadening resulting from the size distribution of the quantum dotsample.

[0192] Crystal Structure of the Core

[0193] For reasons similar to those described above for shape, preferredquantum dots according to some embodiments of the invention will have acore with a crystal structure that is spherically symmetric, morepreferably a cubic or diamond crystal structure. Alternatively, thecrystal structure may be non-spherically symmetric, preferablycylindrically symmetric, more preferably a wurtzite crystal structure.

[0194] It should be recognized that an arbitrary crystal structure maystill be preferred as long as the relative orientation dependence of thebroadening of the linear and nonlinear optical properties is less thanthe broadening resulting from the size distribution of the quantum dotsample. Once again, the considerations described here regarding theimportance of crystal structure constitute an improvement in the use ofquantum dots as a nonlinear material.

[0195] Semiconductor Materials

[0196] There are a variety of preferred quantum dot materials for someembodiments of the current invention. For any given application, thepreferred materials can be determined based on the specific opticalrequirements for that application. Examples of such preferred materialsinclude but are not limited to inorganic crystals of Group IVsemiconductor materials including but not limited to Si, Ge, and C;Group II-VI semiconductor materials including but not limited to ZnS,ZnSe, ZnTe, ZnO, CdS, CdSe, CdTe, CdO, HgS, HgSe, HgTe, HgO, MgS, MgSe,MgTe, MgO, CaS, CaSe, CaTe, CaO, SrS, SrSe, SrTe, SrO, BaS, BaSe, BaTeand BaO; Group III-V semiconductor materials including but not limitedto AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs and InSb;Group IV-VI semiconductor materials including but not limited to PbS,PbSe, PbTe, and PbO; mixtures thereof; and tertiary or alloyed compoundsof any combination between or within these groups, including but notlimited to GeSe, SnS, SnSe, PbS, PbSe, PbTe ZnGeAs₂, ZnSnP₂, ZnSnAs₂,CdSiAs₂, CdGeP₂, CdGaAs₂, CdSnP₂, and CdSnAs₂.

[0197] Quantum dots of many semiconductor materials can be fabricated,at least in part, using a variety of methods. Some preferred syntheticmethods include those described for Group III-V and Group II-VIsemiconductors as described in U.S. Pat. No. 5,990,479 to Weiss et al.,entitled “Organo Luminescent semiconductor nanocrystal probes forbiological applications and process for making and using such probes”and issued on Nov. 23, 1999; U.S. Pat. No. 5,262,357 to Alivisatos etal., entitled “Low temperature thin films formed from nanocrystalprecursors” and issued on Nov. 16, 1993; U.S. Pat. No. 5,505,928 toAlivisatos et al., entitled “Preparation of III-V semiconductornanocrystals” and issued on Apr. 9, 1996; C. B. Murray et al.,“Synthesis and characterization of nearly monodisperse CdE (E=sulfur,selenium, tellurium) semiconductor nanocrystallites, ”J. Am. Chem. Soc.115, 8706 (1993); and in the thesis of C. Murray, “Synthesis andCharacterization of II-VI Quantum Dots and Their Assembly into 3-DQuantum Dot Superlattices” (Massachusetts Institute of Technology,Cambridge, Mass., 1995), the disclosures of which are herebyincorporated in their entireties by reference.

[0198] The fabrication of some types of shells on quantum dots can beperformed using a variety of methods. Preferred methods include thosedescribed in X. Peng et al., “Epitaxial Growth of Highly LuminescentCdSe/CdS Core/Shell Nanocrystals with Photostability and ElectronicAccessibility,” J. Am. Chem. Soc. 119, 7019 (1997) and B. O. Dabbousi etal., “(CdSe)ZnS Core-Shell Quantum Dots: Synthesis and Characterizationof a Size Series of Highly Luminescent Nanocrystallites,” J. Phys. Chem.B 101, 9463 (1997), the disclosures of which are hereby incorporated byreference in their entirety.

[0199] Two preferred materials for use in quantum dots are silicon andgermanium, according to some embodiments of the invention. Both Si andGe have bulk energy gaps that are less than 1.6 eV, making them idealmaterials from which to fabricate quantum dots that exploit quantumconfinement to enhance optical nonlinearities at telecommunicationswavelengths (where photon energies are typically ˜0.8 eV). The idealchemistry of Group IV materials (as discussed below) further solidifiesthese choices.

[0200] In addition, the electron affinity or ionization potential ofGroup IV materials (e.g., Si and Ge) makes them amenable to formingstrong and stable covalent bonds with organic and inorganic surfaceligands, making them ideal for this purpose and for enabling quantumdots that are stable in ambient as well as reasonably extremeenvironmental conditions. The significance of this capability can bebetter appreciated by recognizing that the surfaces of quantum dotscomprised of more ionic materials often require surfactants or ionicspecies to cap, which involves less preferable and weaker van der Waalsbonds, hydrogen bonds, or ionic bonds. Examples of these more ionicquantum dot materials include Group II-VI materials such as CdSe. Thesemore ionic quantum dots often require complex processing to modify theionic quantum dots so as to enable the more desirable covalent bondingbetween the quantum dot surface and surface ligands, e.g., a surfacelayer or layers comprised of a material different than the core quantumdot material typically needs to be added to the ionic quantum dotsurface, wherein the attached surface layer or layers are amenable tocovalent bonding to surface ligands. An example of such a surface layeris one comprised of CdS.

[0201] In addition, the chemical properties of Group IV materials (e.g.,Si and Ge) are such that a stable oxide can be formed that serves toconfine carriers and to passivate the surface to mitigate surface traps.

[0202] In addition, the Bohr exciton is relatively large in Ge (˜12 nm),thus providing a large size range over which the beneficial effect ofquantum confinement, as discussed in various sections herein, arerelevant.

[0203] A Novel Quantum Dot Material

[0204] In one embodiment, the quantum dots are silicon quantum dots orgermanium quantum dots that are surface passivated (or terminated) withan inorganic layer (such as oxides of silicon and germanium) and/ororganic and/or inorganic surface ligands, herein sometimes referred toas SiQDs and GeQDs, respectively. SiQDs and GeQDs as described hereinare novel types of quantum dots that show definitive quantum confinementeffects as manifested by size dependent properties such assize-dependent energy gaps that can be tuned over a very broad range andin particular from the near infrared to the near ultraviolet. Inaddition, SiQDs and GeQDs are stable under a variety of environmentalconditions including ambient (e.g., pressure: ˜1 atmosphere; Gases: ˜70%nitrogen, ˜30% oxygen; Temperature: ˜20-25° C.) for desired periods oftime depending on the specific application. A SiQD and a GeQD can becomprised of a substantially Si core for a SiQD and a substantially Gecore for a GeQD. In addition, the “surface” of the SiQD can be comprisedof Si and inorganic elements such as oxygen and/or organic ligands (R).In addition, the “surface” of the GeQD can be comprised of Ge andinorganic elements such as oxygen and/or organic ligands (R).

[0205] In one embodiment of the invention, a SiQD comprises asubstantially defect free silicon crystal core of diameter betweenapproximately 1 nm and 100 nm, preferably between approximately 1 nm and20 nm, more preferably between approximately 1 nm and 10 nm, while aGeQD comprises a substantially defect free germanium crystal core ofdiameter between approximately 1 nm and 100 nm, preferably betweenapproximately 1 and 50 nm, more preferably between approximately 1 and20 nm. In the case of an inorganic shell surrounding the silicon orgermanium core, this shell typically has a thickness of betweenapproximately 0.1 and 5 nm. One preferred inorganic shell is SiO_(n),for SiQD and GeO_(n) for GeQD with n ranging between approximately 0 and2, preferably ranging between approximately 1.5 and 2, most preferablyranging between approximately 1.8 to 2. The chemical composition of theshell (e.g., relative amounts of Si (or Ge) and O) is potentiallyvarying continuously through a portion of the shell and optionallyvarying discontinuously through a portion of the shell, in which case ncan represent an averaged value within the shell. In the case of organicsurface ligands terminating the surface, the SiQD and GeQD can compriseligand layers comprising organic molecules with a structure R. R can beany one of a variety of hydrophobic, hydrophilic, or amphiphilicmolecules (a list of preferred surface ligands is included below). Thesurface ligands can provide a surface coverage of available silicon (orgermanium) and oxygen binding sites at the surface to provide betweenapproximately 0% and 100% surface coverage, preferably betweenapproximately 20% and 100% surface coverage, more preferably betweenapproximately 50% and 100% surface coverage, more preferably betweenapproximately 80% and 100% surface coverage, with a maximum of one ormore complete layers of surface ligands. R can optionally comprise aplurality of different organic molecules at a plurality of absolute andrelative densities. Finally, a SiQD or GeQD may optionally compriseadditional R-groups that do not interact directly with the quantum dotsurface, but rather indirectly through other R-groups interactingdirectly with the surface. In this case, surface coverage greater than100% is possible.

[0206] It has long been considered that the production of anambient-stable silicon quantum dot or germanium quantum dot with adefined oxide shell could not be achieved due to difficulties in growinga stable and trap-free surface oxide shell Thus, the SiQD and GeQDdescribed herein represent a substantial advance.

[0207] Methods for fabricating SiQDs and GeQDs in accordance with someembodiments of the invention are discussed below. It should be noted,however, that the current invention refers to SiQDs or GeQDs synthesizedby a variety of other methods in addition to those described herein.Some embodiments of the invention encompass various possible variationsof composition of SiQD and GeQDs that could be made while retaining thegeneral characteristics of a substantially crystalline Si or Ge core anda substantially noncrystalline inorganic (e.g., oxide) shell or organicligand layer.

[0208] Method One—“Top Down” Approach

[0209] A general method for the formation of quantum dots of someembodiments of this invention involves a “top down” approach in which“bulk” material is converted to nanostructured material in the form ofquantum dots. In this approach, a form of energy is applied to a form ofa material from which the quantum dot is to be made. The material can bein bulk form, hence the term “top down.” The material in the bulk formis preferably converted to a fine powder, preferably as fine aspossible, more preferably particles in the nanometer size regime (e.g.,between approximately 1 nm and 100 nm) comprised of the material fromwhich the quantum dot is to be made. One advantage of using a finepowder is that it leads to shorter processing times to achieve thedesired quantum dot. The applied energy can be in the form of, forexample, acoustic or vibrational energy (e.g., sound energy), opticalenergy (e.g., light energy), electrical energy, magnetic energy, thermalenergy, chemical energy, or any combination thereof. More precise sizecontrol of the final quantum dots can be achieved by applying more thanone source of energy to the starting material. Multiple sources ofenergy can be applied simultaneously or they can be applied in varioussequential combinations. This also leads to shorter processing times toachieve the desired quantum dot. It is believed that the applied energyfractures or breaks the starting material into smaller particles and/or“grows” the starting material into larger particles by, for example,fracturing or breaking (e.g., “consuming”) the smaller particles. Inessence, this method evolves the starting material into the desirednanostructured form. This results in the unique quantum dots describedherein in which a stable, well-formed, substantially defect-freeinorganic shell is formed on the surface of the QD, as well as adefect-free interfacial region between the core and the shell. Thisshell imparts a high stability to the QDs under a variety ofenvironmental conditions including ambient. In the specific case of anoxide shell, the oxide is stable and substantially defect-free.

[0210] Specific examples of “top-down” methods of formation of quantumdots of some embodiments of this invention follow:

EXAMPLE 1 Oxide-Terminated SiQDs

[0211] A powdered form of Si, from which Si quantum dots can made forsome embodiments of this invention, is derived from porous silicon(PSi). A nanostructured PSi layer is removed and made into a finepowder. Energy is then applied in the form of sound energy throughsonication and light energy through irradiation with a light source. Thesize of the SiQDs is determined by the duration and power of thesonication (with longer and higher power sonication giving rise tosmaller quantum dots) and by the characteristics of the light source(with shorter wavelengths and longer irradiation times giving rise tosmaller quantum dots).

[0212] The method in this example uses sonication periods sufficient toform stable and well-formed quantum dots with stable oxide surfacetermination that were not previously available. In addition, the methodallows the size of the quantum dots to be controlled by the sonicationperiod. In addition, the method also uses light irradiation as a meansto control the size and the size distribution of the quantum dots, whereshorter wavelengths of irradiation and longer irradiation times givesmaller quantum dots and narrower size distributions. This lightirradiation allows better control over the size and range of sizes ofthe quantum dots than previously available, with sizes ranging from ˜1nm to ˜6 nm in diameter in one embodiment of the invention.

[0213] The result from the method of this invention is oxide-terminatedSiQDs that are stable under a variety of environmental conditionsincluding ambient. This stability results largely from the stable andsubstantially defect-free oxide shell and interfacial region between thecore and shell.

[0214] PSi is formed using a variety of methods that include, but arenot limited to, anodic electrochemical etching of p-doped or n-dopedsilicon as, for example, described in A. G. Cullis et al., “Thestructural and luminescence properties of porous silicon,” J. Appl.Phys. 82, 909 (1997), the disclosure of which is incorporated herein byreference in its entirety. One preferred method includes starting withp-type (e.g., Boron-doped) silicon (Si) wafers comprising a plurality oforientations, with the (100) orientation being preferred. The waferresistivity preferably ranges from 0.02 Ω-cm to 30 Ω-cm. The wafer ispreferably between approximately 500-600 microns thick. Electricalcontact to the wafer is made through a thin layer of metal (e.g.,aluminum or platinum; preferably between approximately 100-500 micronsthick) deposited on the backside of the wafer. Anodic electrochemicaletching is performed on the wafer, which is placed in a solutioncomprising aqueous hydrofluoric acid (HF, preferably 48 wt %) andethanol. The weight percentage of ethanol to aqueous HF ranges betweenapproximately 0% and 60%, preferably between approximately 45% and 55%.Various conducting materials can be used as the counter electrode inwhich metals are an example. Examples of such metals include, but arenot limited to, aluminum, copper, brass, and platinum.

[0215] The metal layer making electrical contact with the silicon wafermay optionally be protected from erosion in the acidic solution byisolating the metal layer from the solution. This can be achieved bysealing the silicon surface with a gasket such that the etching solutionis substantially only in contact with the silicon side of the substrate.Alternatively, the electrode metal can be selected to be relativelyinert under the selected etching conditions or can be selected with athickness great enough to withstand the etching procedures describedbelow.

[0216] Electrochemical etching of the Si wafer is carried out forvarious time durations (which can range from between approximately 2 and200 minutes, depending on the starting parameters) using a constantcurrent density ranging from between approximately 5 and 1000 mA/cm²with approximately 60 mA/cm² as a preferred current density andapproximately 30 minutes as a preferred etching time. After etching, thesurface of the Si wafer is left with a thin layer (between approximately10 microns and 1 mm in thickness) of nanostructured material, whichcomprises PSi. The peak of the luminescence of the PSi ranges typicallyfrom 600 nm to 800 nm (or to greater than 0.800 nm).

[0217] The PSi is optionally rinsed with deionized water, dried under astream of nitrogen gas, and placed in a vacuum chamber. The chamber isevacuated to a moderate pressure for several hours, preferably less than1 Torr, more preferably less than 500 mTorr, and most preferably lessthan 100 mTorr. The samples are then transferred to a solvent-freeenvironment (e.g., a drybox). The nanostructured PSi layer is thenmechanically removed or scraped (which can be accomplished, for example,with a knife edge or scalpel) from the Si substrate, and the removedmaterial is collected. The nanostructured PSi layer can also beseparated from the Si wafer through a second electrochemical etchingprocess in which low concentration HF/H₂O (preferably between 0.5 and2%) and a high current density (preferably greater than 160 mA/cm²) isused for a few minutes to separate the anodized and nanostructured PSilayer from the Si substrate.

[0218] After the PSi layer has been separated from the siliconsubstrate, the PSi is ground into a fine powder (using, for example, amortar and pestle and/or a mechanical agitator) yielding about 25 to 40mg of powdered PSi from a wafer surface area measuring approximately 1inch in diameter. The peak of the PL spectra of the powder is in the redspectral region and ranges any where from 600 nm to greater than 800 nm,depending on the conditions of the electrochemistry. A solvent is thenadded to the powdered PSi. Preferred solvents include, but are notlimited to, acetonitrile, toluene, hexane, methanol, ethanol, ethyleneglycol, and water. In the case of organic solvents, the solvent may bedried over a dehydrant (e.g., calcium hydride or magnesium sulfate),distilled, and degassed prior to being added to the PSi.

[0219] The resulting mixture of PSi powder and solvent is placed in abath and sonicated with acoustic waves or sound energy for a period oftime. Although acoustic energy is being disclosed, it is to beunderstood that other types of energy may be used, as discussed above.The sonication can be accomplished with a variety of equipment thatemits acoustic waves or vigorously agitates or shakes the powder, withan ultrasonic bath being a particularly convenient method.

[0220] The size and size distribution of the quantum dots in the mixturecan be controlled by varying the duration of sonication. The preciseperiod of time required for sonication depends on a number of factorsthat include the acoustic power of the sonicator, the solvent used, theinitial size and size distribution of the nanostructures in the PSipowder, etc., and the characteristics of the sonication should becalibrated for the specific processing conditions used. A factordetermining the optimum time duration of the sonication is the timerequired to achieve the desired size of the resultant quantum dots,i.e., the sonication is continued until the desired quantum dot size isreached (e.g., until cores are formed with diameters within apredetermined or desired range). Generally, the size of the quantum dotsdecreases as the sonication time increases, and the size of the quantumdots can be determined by the energy gap or the peak wavelength of thephotoluminescence of the colloidal suspension of quantum dots. Therelationship between quantum dot size and energy gap and therelationship between the PL peak wavelength and quantum dot size forsilicon quantum dots were previously described with respect to FIGS. 2and 3. Therefore, the photoluminescence spectrum of the colloidalsuspension can be periodically taken during the sonication process tomonitor the progress of the sonication. Typically, the PL peakwavelength shifts towards shorter wavelengths (corresponding to a shifttowards smaller peak sizes) during the sonication process. Once thesonication time is calibrated for the processing conditions used to givethe desired quantum dot size, this method can give very consistentresults.

[0221] Another factor that can be used to determine the optimum timeduration of the sonication is the time required to achieve the desiredshells for the resultant quantum dots, e.g., the sonication is continueduntil oxide shells are formed having desired properties as discussedherein. If desired, the sonication time can be calibrated for theprocessing conditions used to give the desired photoluminescence quantumefficiencies.

[0222] As mentioned above, the precise relationship between sonicationtime and the quantum dot size that results depends on several parametersthat may need to be calibrated with each specific fabrication setup andconditions. The following is an example that serves as a point ofreference. With a sonication power of 80 W and with methanol as thesolvent, a sonication period of 10 days resulted in oxide-terminated Siquantum dots with an average size of ˜1.5-1.7 nm in diameter andemitting in the near ultraviolet-blue; a sonication period of 3 daysresulted in quantum dots with an average size of ˜2.5 nm in diameter andemitting in the green; a sonication period of 1 day resulted in quantumdots with an average size of ˜3.6 nm in diameter and emitting in thered.

[0223] Upon removal from the ultrasonic bath, the mixture is allowed tosettle and is centrifuged, and the supernatant is filtered to remove anylarge particles. Preferred pore sizes of the filter range betweenapproximately 20 m and 450 nm. Filters can also be used to separate thedifferent sizes of quantum dots. Additionally, other separationtechniques such as chromatography, more specifically gel permeationchromatography or size exclusion chromatography, can be used to separatethe different sizes of quantum dots. The result is a colloidalsuspension of oxide-terminated Si quantum dots (SiQDs) of various sizesthat are stable in a variety of environmental conditions that includeambient (room temperature, pressure, and atmosphere).

[0224] As mentioned above, more precise size and size distributioncontrol of the final quantum dots can be achieved by applying more thanone source of energy to the starting material. In one preferred method,two sources of energy are applied to the starting material. Oneadditional preferred source of energy is light energy. In this exampleof a preferred method, the sample is irradiated with light during orsequentially with sonication. The light source can be a lamp (e.g.,Tungsten, Xenon, or Mercury), a light emitting diode (LED), a laser, orany other light source capable of emitting light at the appropriatewavelengths, where “appropriate wavelengths” is described below.Alternatively, irradiation can be implemented during the electrochemicaletching process (in which the etched surface of the Si wafer isirradiated). The size of the quantum dots that result is determined by anumber of parameters including wavelength, intensity, spectralbandwidth, and duration of irradiation. Preferably, the wavelength ofirradiation should be within the spectral region where the light isabsorbed by at least a subset of the quantum dot sizes to be controlled.Specifically, within a size distribution of quantum dots, the longer thewavelength of the irradiation, the larger the size of the resultingquantum dots. More specifically, to achieve a specific size SiQD, thesample should be irradiated with photons of energy approximately equalto the energy gap of the desired SiQD. This effect can be accentuated byincreasing the duration and/or intensity of irradiation. In particular,the size and size distribution of the quantum dots in the mixture can becontrolled by varying the duration of irradiation. The optimum timeduration of the irradiation is the time required to achieve the desiredsize and/or size distribution of the resultant quantum dots, i.e., theirradiation is continued until the desired quantum dot size is reached(e.g., until cores are formed with diameters within a predetermined ordesired range) and/or until the desired size distribution is reached(e.g., until substantially monodisperse quantum dots are formed).

[0225] For any specific set of synthesis parameters, the preciserelationship between irradiation wavelength, irradiation intensity,irradiation duration, and quantum dot size should be calibrated as isdone in the case of the sonication method alone. This can be achieved bymonitoring the energy gap or peak wavelength or spread of thephotoluminescence at various times during the irradiation as anindicator of the progress toward the desired quantum dot. Typically, thephotoluminescence has a peak wavelength that shifts towards shorterwavelengths (corresponding to a shift towards smaller peak sizes) and awavelength spread that narrows (corresponding to a shift towardsnarrower spread in sizes) during the irradiation process.

[0226] The following serves as examples or points of reference.Simultaneously irradiating and sonicating the sample as described abovefor 5 days with 50 mW of laser light at 400 nm results inoxide-terminated SiQDs that luminesce in the near ultraviolet-bluespectral region; simultaneously irradiating and sonicating the sample asdescribed above for 2 days with 100 mW of laser light at 532 nm resultsin oxide-terminated SiQDs that luminesce in the green spectral region;simultaneously irradiating and sonicating the sample as described abovefor 0.5 days with 150 mW of laser light at 620 nm results inoxide-terminated SiQDs that luminesce in the red spectral region.

[0227] The result of this “top down” approach is oxide-terminated Siquantum dots that are stable in a variety of environmental conditions,including ambient. This capability was previously thought to not bepossible. This is achieved in the “top down” approach through theestablishment of a stable and substantially defect-free silicon oxideshell surrounding the Si quantum dot core.

[0228] The defect-free nature of the resulting SiQDs is manifested inthe quantum efficiency of the photoluminescence from these SiQDs. Thepresence of defects in quantum dots can trap excited carriers (electronsand holes). These trapped carriers can either nonradiatively relax, orthey can radiatively recombine in a defect. Both processes lead to a lowquantum efficiency for the photoluminescence from the quantum dots.Previous quantum dots formed of Si or Ge typically exhibitedphotoluminescence quantum efficiencies of ˜1-5%. In contrast, thephotoluminescence quantum efficiency of the SiQDs made with the methodsof some embodiments of this invention is greater than 6%, preferably atleast or greater than 10%, more preferably at least 20%, more preferablyat least 30%, more preferably at least 40%, and more preferably at least50% (e.g., as high as between approximately 50% and 60%). Thisrepresents the largest photoluminescence quantum efficiency observed forsuch quantum dots.

[0229]FIG. 3 shows global PL spectra of six samples of different sizedoxide-terminated Si quantum dots made with the method described herein.FIG. 3 shows that the light emission can be readily tuned from the redto the ultraviolet, and as a result, the size can be readily tuned aswell. And, this light emission is stable in a variety of environmentalconditions including ambient.

[0230] The electronic and optical properties of these SiQDs that aremade in this fashion are unique in that they show size dependentproperties that are uniquely consistent with quantum confinement. Theoptical and electronic properties of these Si quantum dots are uniquelyconsistent with theoretical calculations more sophisticated thanEffective Mass approaches, such as the Empirical Pseudopotential Methodand the Tight Binding Method. A comparison of the size dependent energygap calculated by these methods and with measurements taken on the SiQDssynthesized by the method disclosed herein is shown in FIG. 2. Theagreement is extremely good and is the best observed for any quantumdots formed of Si.

[0231] The use of light to control the physical size or sizedistribution of the quantum dots in this synthetic process is aparticularly novel aspect of certain embodiments of the presentinvention. Previous methods have typically used optical excitation notfor control of the physical parameters of the quantum dots but toinitiate the chemical reaction needed for quantum dot formation, i.e.,to photolyze the chemical precursors. As described above, embodiments ofthe invention utilize optical control over the physical parameters ofquantum dots in a synthetic method. This aspect is also applicable toother quantum dot synthetic procedures and is not limited to thosedescribed herein with respect to SiQDs and GeQDs.

[0232] Overall, the size and size distribution of the resulting SiQDscan be precisely controlled by varying the duration of sonicationprocessing, the strength or intensity of the acoustic energy of thesonication device, the photon wavelength (photon energy) of irradiation,the intensity of irradiation, the spectral width of irradiation, theduration of irradiation, the size and size distribution of the startingmaterial, and the solvent into which the starting material isincorporated. For certain embodiments of the invention, the average SiQDsize can be varied from ˜1 nm to greater than 6 nm with this technique.These average sizes give rise to light emission from the infrared to theultraviolet.

[0233] Alternatively, PSi can be fabricated using n-doped Si wafers. Inthis case, a process similar to that described above can be followed.However, the electrochemical etching process may be performed in thedark, and the wafer desirably should be illuminated with a light source(UV light being preferred) during etching in order to generate “holes”(as opposed to electrons) needed in the etching process.

EXAMPLE 2 Oxide-Terminated GeQDs

[0234] In the synthesis above, the reactants and starting materials canbe replaced with their germanium counterparts for the formation ofGeQDs. As shown in FIGS. 4(a) and 4(b), GeQDs of sizes ranging from 1 nmto 16 nm have been synthesized using the method according to anembodiment of the invention.

[0235] The electronic and optical properties of these GeQDs that aremade in this fashion are unique in that they show size dependentproperties that are uniquely consistent with quantum confinement. Theoptical and electronic properties of these GeQDs are uniquely consistentwith theoretical calculations more sophisticated than Effective Massapproaches, such as the Empirical Pseudopotential Method and the TightBinding Method. A comparison of the size dependent energy gap calculatedby these methods with measurements taken on the GeQDs synthesized by themethod disclosed herein is shown in FIG. 4(a). The agreement isextremely good and is the best observed for any quantum dots formed ofGe.

[0236] Method Two—“Bottoms Up” Approach

[0237] In another embodiment, quantum dots can be fabricated fromchemical precursors. This is essentially a “bottoms up” approach inwhich the quantum dots can be assembled “atom-by-atom” through chemicalsynthesis.

[0238] The present invention provides general high yield methods ofsynthesizing surface-functionalized quantum dots and, in particular,methods of synthesizing soluble quantum dots of a Group IV semiconductormaterial in a solution at relatively low temperatures.

[0239] The methods can be described by reference to the following:

[0240] Method 2-a:

YX_(a)+Reducing Agent→(Y)X  (1)

(Y)X+Capping Agent(R)→(Y)R  (2)

[0241] Method 2-b:

YX_(a)+R_(b)YX_(c)+Reducing Agent→R(Y)X  (3)

R(Y)X+Capping Agent(R′)→R(Y)R′  (4)

[0242] wherein YX_(a) is a source of Y, with Y being Si or Ge, and X isselected from the group consisting of —F, —Cl, —Br, —I, —O—CO—R⁽¹⁾,—NR⁽²⁾R⁽³⁾, —O—R⁽⁴⁾, —S—R⁽⁵⁾, and so forth, with R⁽¹⁾, R⁽²⁾, R⁽³⁾, R⁽⁴⁾,and R⁽⁵⁾ independently selected from the group consisting of alkyls,alkenyls, alkynyls, aryls, and so forth. The reducing agent is selectedfrom either activated metals (e.g., Group IA, Group IIA, transitionmetals, and lanthanides) or hydrides (Group IIIB hydrides, Group IVBhydrides, and transition metal hydrides). Electrochemical reduction canalso be used for reduction. The capping agent(R) and the cappingagent(R′) are sources of surface ligands R and R′, respectively, and canbe selected from organometallic reagents, e.g., RM (or R′M), with R (orR′) being a surface ligand (e.g., a linear or branched alkyls, alkenyls,alkynyls, ether, ester, acid, amide or nitrile moiety having between 1and about 20 carbon atoms). It should be recognized that the surfaceligands R and R′ can be the same or different. The capping agents canalso be an alcohol, amine, thiol, and so forth. M is preferably fromGroup IA, Group IIIA, or Group IIB. In the above, “a” represents anoxidation state or coordination number of Y in the source of Y, which istypically 2, 4, or 6, and “b” and “c” are integers that can each rangefrom 1 to 6. “a” is typically equal to the sum of “b” and “c”. In method2-a, (Y)X represent intermediate particles comprising cores including Yand with surfaces terminated with X, and (Y)R represent quantum dotsthat are formed with surfaces terminated with R. In method 2-b, R(Y)Xrepresent intermediate particles comprising cores including Y and withsurfaces terminated with R and X, and R(Y)R′ represent quantum dots thatare formed with surfaces terminated with R and R′.

[0243] The basic strategy involves solution phase reduction of Si^(a+)or Ge^(a+), where a represents the oxidation state of Si or Ge, andsubsequent termination with organic or organometallic reagents. Themethods according to some embodiments of the invention allow mildsynthesis, precise manipulation, functionalization, and interconnectionof the Group IV quantum dots to an extent not previously achieved. Thekey differentiations between previously used methods and the methodsaccording to some embodiments of the invention include one or more ofthe following:

[0244] Some embodiments can avoid arduous procedures typicallyassociated with the use of highly pyrophoric and air-sensitive startingmaterials, such as Group IV Zintl compounds or sodium metal. The GroupIV Zintl salts are typically prepared by combining starting materials(e.g., K and Si) at elevated temperature (500-900° C.) in a sealed tubefor a few days. As an example, a method of some embodiments of thisinvention uses milder and air-stable reducing agents such as magnesium(Mg), other Group IIA metals, transition metals, or lanthanides. Thismakes the method more amenable to scale up and large scale manufacture.

[0245] Some embodiments provide a method in which the reactionconditions are less extreme than required by previous methods. Inparticular, a method of some embodiments of the invention avoids thehigh pressure and high temperature conditions as sometimes previouslyused that can produce large amounts of undesirable insoluble materials.

[0246] Some embodiments need not utiltize high energy sonochemicaltechniques for reduction of Si⁴⁺, which has typically produced eithersmall amorphous particles with ill-defined surface composition or largerinsoluble aggregates with an irregular network.

[0247] Some embodiments need not utilize highly toxic gaseous Group IVhydrides and pyrophoric metal hydrides.

[0248] The yields from the method of some embodiments in this inventionare significantly higher than in previously reported methods. In someembodiments, the yields that can be obtained are between approximately35% and 95%.

[0249] The size control that can be achieved is greater than previousmethods.

[0250] The range of sizes possible that can be produced is greater thanachievable with previously reported methods. Some embodiments allowproduction of different sizes of quantum dots that can give rise toinfrared to ultraviolet light emission (e.g., not limited to productionof smaller quantum dots that emit primarily in the blue and blue-greenregion).

[0251] The resultant quantum dots are not limited to certain sizedistributions (e.g., the size distribution control that can be achievedis greater).

[0252] Some embodiments afford quantum dots with defined surfacecomposition and high surface coverage with surface ligands.

[0253] The resultant quantum dots are more stable than those producedfrom other methods and have the unique properties as described herein.

[0254] The resultant quantum dots are more crystalline than thoseproduced from other methods.

[0255] Quantum dots can be produced with higher amounts than achievablewith other methods. (e.g., in quantities of at least ten grams).

[0256] The functionalization of quantum dots using methods 2-a or 2-ballows functional group inter-conversion at the surface of intermediateparticles produced in equations 1 or 3 with appropriate organic reagentsin equations 2 or 4 to form ligands layers. The intermediate particlestypically will comprise cores that include Y. The organicfunctionalization of these quantum dots imparts favorable solubility incommon organic solvents and compatibility in various matrix materialssuch as organic polymers, inorganic polymers, gels, glasses, and thelike.

[0257] Method 2-a is based on controlled chemical reduction of readilyavailable molecular Si^(a+) and Ge^(a+) reagents, where a ranges from 2to 6 and typically from 2 to 4, and the quenching of the correspondingintermediate particles (Y)X with different reagents in a reactionmedium. Two suitable families of reducing agents are activated metalsand hydrides. A nonaqueous reaction medium desirably should be used forthe reduction of silicon and germanium reagents because of the largenegative reduction potential and high oxophilicity of Group IVcompounds. The controlled addition of a capping agent(R) such as RM(R=alkyl, aryl, etc. and M=Li, Na, MgA, ZnA, with A being a halogen, andso forth) to the corresponding intermediate particles at relatively lowtemperatures produces the functionalized quantum dots in high yield.

[0258] In a further effort to control quantum dot particles and sizedistributions, terminating agents, R_(b)YX_(c), can be employed usingmethod 2-b (equation 3). R group, in this method, serves as theterminating agent for the quantum dots. The ratio of R_(b)YX_(c), to theYX_(a) reagent in equation 3 can be used as a basic measure of thesurface-to-volume ratio of the quantum dots. A mixture of YX_(a) andR_(b)YX_(c), in the presence of a reducing agent, yields thecorresponding intermediate particles, R(Y)X, which can be treated with acapping agent(R′) such as R′M to displace the remaining leaving groups,X, on the surface of the intermediate particles (equation 4). Thismethodology can, in principle, produce highly functionalized quantumdots.

[0259] A preferred method of this chemical synthetic method is describedas follows.

[0260] A silicon source, e.g., SiCl₄, is reacted with a reducing agent,e.g., Mg powder, under an inert atmosphere, e.g., argon. These materialsare heated together in a liquid-phase reaction medium. The reactionmedium should desirably be aprotic. It can be a hydrocarbon, or it couldbe aromatic. It could be a cyclic or acyclic ether, an aromatic ether,or a polyether. It could contain oxygen, nitrogen, sulfur, and/orphosphorous (so long as it is compatible with the other reagents). Itcan include an organic solvent with various combinations of more thanone hetero-atom or any combination of the solvents discussed previously.Representative solvents include alkanes such as heptane, decane, andoctadecane; aromatics including benzene, tetralin, and naphthalene; andalkylaromatics such as toluene, xylene, and mesitylene; ethers such asdialkylethers, diarylethers, alkylarylethers, and cyclic ethers; andpolyethers like glymes.

[0261] In this process, a Group IV source, such as providing Si⁴⁺, Ge⁴⁺,or Ge²⁺, especially in the form of halides or with corresponding 1-20carbon organic substituent (RA, R=organic substituent, A=O, S, N, Si,etc.), is reacted with a reducing agent, such as a Group IA compound,Group IIA compound, transition metal, lanthanide, or hydride, in liquidphase reaction medium at an elevated temperature. Representative GroupIV sources include SiF₄, SiCl₄, SiBr₄, SiL₄, GeF₄, GeCl₄, GeBr₄, GeI₄,GeCl₂, GeBr₂, GeI₂, SiR₄, Si(OR)₄, Si(SR)₄, Si(NR⁽¹⁾R⁽²⁾)₄, Si(O₂R)₄,Si(SiR)₄, GeR₄, Ge(OR)₄, Ge(SR)₄, Ge(NR⁽¹⁾R⁽²⁾)₄, Ge(O₂R)₄, Ge(SiR)₄,Ge(NR⁽¹⁾R⁽²⁾)₂ as well as the dimmers and the higher oligomers of theabove reagents (R, R⁽¹⁾, R⁽²⁾=organic substituent). Representativereducing agents include Li, Na, K, Na/K alloy, Rb, Cs, Be, Mg, Ca, Sr,Ba, Sc, Ti, Zr, Mn, Fe, Co, Ni, Pd, Cu, Zn, Ce, Sm, Gd, Eu, LiAlH₄,NaBH₄, Super-hydride, L-Selectride, RSiH₃, R₂SiH₂, R₃SiH (R=organicsubstituent), and the like. Reducing agents can be provided in a varietyof forms (e.g., as a powder, a liquid, a solid, and so forth). Forcertain reducing agents (e.g., a Group IIA compound such as, forexample, Mg), it is desirable to provide such reducing agents in apowdered form to facilitate reaction with the Group IV source.Alternatively, or in conjunction, it is desirable to provide suchreducing agents in other forms such as in the form of chips, a mesh,dendritic pieces, ribbons, rods, turning or activated (e.g., “Riekemagnesium”, etc.).

[0262] One or two of each of these two groups of materials are mixedtogether in the reaction medium (e.g., an anhydrous aprotic solvent) forat least few minutes. For some embodiments, the reaction between asource of Si or Ge and a reducing agent is performed by maintaining thereaction medium at a temperature between approximately −78° C. and 300°C., preferably between approximately 60° C. and 280° C., and at aroundambient pressure (e.g., about 1 atm) for a period of time betweenapproximately 2 and 48 hrs. For some embodiments, the reaction betweenNa and silicon reagents can require an elevated temperature and aprolonged period to complete. The reflux temperature of the reactionmedium can be used. Elevated pressures of up to about 100 atmospherescan be used to obtain higher temperatures. Suitable temperatures rangebetween approximately 25 and 300° C.

[0263] In an additional step in the same pot, the intermediate product,which is chemically labile, can be functionalized with organicsubstituents when treated with an appropriate reactive material (e.g.,capping agent, surface ligands, molecular tethers, terminating agents,passivator, etc.). These reagents can be organometallic reagents, RM(R=surface ligand such as alkyl, aryl, heteroaryl, and so forth andM=Li, Na, MgA, ZnA, with A being a halogen, and so forth), alcohols,amines, amides, thiols, phosphines, oxyphosphines, acids, silanes,germanes, oxides, silanols, and germanols or their corresponding anionsalts. Representative ligand sources include organolithium reagents(e.g., n-butyllithium, sec-butyllithium, tert-butyllithium,n-hexyllithium, and phenyl lithium); Grignard reagents (e.g.,octylmagnesium halide, phenylmagnesium halide, and allylmagnesiumhalide); alcohols (e.g., ethanol, isopropyl alcohol, and phenol); aminesand thiols (e.g., diethylamine, octylamine, and hexylthiol); and thelike.

[0264] For some embodiments, the reaction between the intermediateparticles or nanocrystallites and the source of surface ligands canrequire a prolonged period to complete and can require an elevatedtemperature. Suitable temperatures range from room temperature to about100° C. The reaction can be completed in between approximately 2 and 100hours at ambient temperature. Subsequent work-up affords the organicallyfunctionalized quantum dots as a powder. The subsequent work-uppreferably involves the addition under an inert atmosphere (e.g. argon)of acidic water to destroy the unreacted reducing agent or theorganometallic reagent. The product can be extracted with organicsolvents. The solvent can be a hydrocarbon, an aromatic, or a mixedhydrocarbon fraction. It could be an ether or a polyether. It could beester. It could contain nitrogen, sulfur, and/or halides (so long as itis not very soluble in water). Representative solvents include hexanes,decane, toluene, xylene, diethyl ethers, glyme, dichloromethane,chloroform, ethyl acetate, carbon disulfide, and the like. Theextraction process desirably should be repeated several times to improvethe yield.

[0265] The product is a quantum dot powder that can be isolated byremoving the solvent. This can be carried out by evaporation,filtration, and the like.

[0266] The synthetic method described above is associated with yields inthe range of 35 to 95%, which are significantly higher than previouslyobtainable. A broad range of particle sizes can be achieved, e.g.,between approximately 1-100 nm.

[0267] Various factors can affect particle size, including the nature ofthe reaction medium, the nature of the reducing agent, the nature of thestarting material, the ratio of reagents employed, concentration,temperature, and pressure employed. The reaction medium employed canplay an important role in the physical properties of the quantum dotproduct. More particularly, coordinating solvents or agents such asoxygen or nitrogen or sulfur or phosphorous containing organic compoundstend to yield quantum dots with larger particle size. In particular, thesize and the size distribution of the quantum dots can be controlled byvarying the coordination ability of the solvent or the co-solvent. As tothe effect of temperature, higher reaction temperatures improve thecrystallinity of the quantum dots and aid in the production of biggerquantum dots. Concentration also affects the particle size, with lowerconcentrations tending to produce smaller quantum dots (the largeramount of solvent effectively causes better heat dispersion from thereacting species).

[0268] This invention will be further described by the followingexamples. These examples are not to be construed as limiting the scopeof this invention, which is defined by the appended claims.

EXAMPLE 1

[0269] A 500-ml three-neck round bottom flask equipped with a stirringbar, a reflux condenser, and a thermometer was purged with argon andcharged with 200 ml of the selected solvent (e.g., glymes (n=1 to 5))and the reducing agent (e.g., magnesium powder, 0.05 to 0.20 mol).Freshly distilled YX₄ (0.05 to 0.20 mol) was added dropwise, and theresulting brown-reddish solution was heated to higher temperatures(e.g., between approximately 60 and 280° C.) for a period of time (e.g.,between approximately 2 and 100 hrs, typically between approximately 2and 48 hrs). The resulting mixture was cooled to about −20° C. andtreated with an excess amount of the capping agent (e.g., 1.8 M solutionof phenyllithium), which was added dropwise to keep the temperaturebelow room temperature. After the reaction mixture was stirred atambient temperature for a period of time (e.g., between approximately 2and 48 hrs), it was quenched with dilute protic acid (pH˜2) andextracted with an organic solvent (e.g., toluene). The combined organicextracts were washed with water and dried over a drying agent (e.g.,sodium sulfate). The solvents were removed under reduced pressure, andtraces of the solvents were removed by precipitation with a nonsolvent(e.g., pentane). After centrifugation or filtration, the product wascollected and dried in a vacuum oven. The product can be purified bycolumn chromatography (e.g., silica, CH₂Cl₂/methanol, 95/5).

EXAMPLE 2

[0270] The preparation of Example 1 is repeated using sodium as thereducing agent.

EXAMPLE 3

[0271] The preparation of Example 1 is repeated using barium as thereducing agent.

EXAMPLE 4

[0272] The preparation of Example 2 is repeated using a mixture of35%/65% (by volume) diglyme/xylenes as the reaction medium.

EXAMPLE 5

[0273] The preparation of Example 1 is repeated using diphenyl ether asthe reaction medium.

EXAMPLE 6

[0274] The preparation of Example 1 is repeated using tetraglyme as thereaction medium.

EXAMPLE 7

[0275] The preparation of Example 1 is repeated using n-butyllithium asthe capping agent.

EXAMPLE 8

[0276] The preparation of Example 1 is repeated using 3-butenylmagnesiumbromide as the capping agent.

EXAMPLE 9

[0277] The preparation of Example 1 is repeated using allylmagnesiumbromide as the capping agent.

EXAMPLE 10

[0278] The preparation of Example 1 is repeated using4-methoxyphenylithium as the capping agent.

EXAMPLE 11

[0279] The preparation of Example 1 is repeated usingpentafluorophenyllithium as the capping agent.

EXAMPLE 12

[0280] The preparation of Example 1 is repeated usingperfluorohexyllithium as the capping agent.

EXAMPLE 13

[0281] The preparation of Example 1 is repeated using sodium ethoxide asthe capping agent.

EXAMPLE 14

[0282] The preparation of Example 1 is repeated using silicontetrabromide as the source of silicon.

EXAMPLE 15

[0283] The preparation of Example 1 is repeated in a sealed pressurereactor at 260° C.

EXAMPLE 16

[0284] The preparation of Example 1 is repeated using a mixture of70%/10%/10%/10% (by molar ratio) germaniumtetrachloride/phenyltrichlorogermane/diphenyldichlorogermane/triphenylgermanium asgermanium source and terminating agents, R_(b)YX_(c).

[0285] Preferred Surface Ligands and Molecular Tethers

[0286] As described in the previous sections, the ligand layer can serveto passivate the surface of a quantum dot and eliminate surface defects.It also facilitates compatibility with matrix materials. This is furtherexplained as follows. Fluoropolymers are a group of desirable materialsfor optical applications because of their unique properties.Fluoropolymers, in general, have low indices of refraction (e.g., incomparison with regular hydrocarbon polymers) and thus low intrinsicscattering loss. They also, in general, exhibit low absorption loss asthey are typically comprised of little or no carbon-hydrogen bonds. Theyare hydrophobic and thus low in moisture absorption. They, in general,are chemically and thermally inert and thus compatible in demandingenvironments and extreme process conditions in device fabrication.Because of their inertness, fluoropolymers are nearly non-mixable withmany materials, such as conventional quantum dots. Embodiments of thecurrent invention provide a novel approach to circumvent thecompatibility issue by introducing fluorinated surface ligands to thesurface of quantum dots (e.g., as in Example 11 and 12 of the precedingsection). The quantum dots, which are terminated with a ligand layer offluorinated surface ligands, can now be incorporated into, for example,Cytop® brand polymer (a perfluorinated polymer from Asahi), facilitated,for example, by using a solvent vehicle, CT-SOLV 180 from Asahi.

[0287] The following are preferred surface ligands of the ligand layer,according to some embodiments of the invention. This list, which is notintended to be exhaustive, describes a number of surface ligands havingdesirable physical characteristics that can be used to form ligandlayers for SiQDs or GeQDs. In the following, Y is Si or Ge, and Y—C,Y—O, Y—S, Y—Si, and Y—N denote covalent bonds between Si or Ge and a Catom, an O atom, a S atom, a Si atom, and a N atom, respectively. Otherpreferred surface ligands, not listed below, can contain a P or a Seatom that is covalently bonded to Si or Ge.

[0288] Y—C

[0289] A) Alkyls

[0290] a. Simple aliphatic alkyl groups (e.g., methyl, ethyl, propyl,etc.)

[0291] b. Branched and cyclic alkyl groups (e.g., iso-propyl,tert-butyl, cyclohexyl, etc.)

[0292] c. Substituted alkyl groups (e.g., 4-cyanobutyl,3-ethoxy-3-oxopropyl, etc.)

[0293] d. Perfluorinated alkyl groups (e.g., linear, branched, orcyclic)

[0294] B) Alkenyls

[0295] a. Simple isolated double bonds (e.g., 1-hexenyl, 1-dodecenyl,etc.)

[0296] b. Substituted alkenes (e.g., 6-heptenenitrile, etc.)

[0297] c. Conjugated polyenes (e.g., pentadienyl etc.)

[0298] d. Polymerizable alkenes (e.g., allyl, 3-butenyl, 2-butenyl etc.)

[0299] C) Alkynyls

[0300] a. Simple isolated alkynes (e.g., hexynyl, octynyl, etc.)

[0301] b. Substituted alkynes (e.g., phenylethynyl, etc.)

[0302] c. Polymerizable alkynyls

[0303] d. Perfluoro alkynyls

[0304] D) Aromatics and Aromatic Heterocycles

[0305] a. Phenyls, Pyridyls, Thienyl, etc.

[0306] b. Substituted Aromatics and Aromatic Heterocycles

[0307] i. With electron withdrawing groups (nitro, nitrile, fluoro,perfluoro, carboxylate, e.g., 4-cyanophenyl, etc.)

[0308] ii. With electron donating groups (amino, alkoxy, e.g.,4-methoxyphenyl, etc.)

[0309] E) Conjugated Aromatics, Aromatic Heterocycles, and Polyenes(poly is referred to well defined oligomers)

[0310] a. Polyenes

[0311] b. Poly(p-phenylene)

[0312] c. Poly (diacetylene)

[0313] d. Poly(triacetylene)

[0314] e. Poly(p-phenylene vinylene)

[0315] f. Poly(p-phenylene ethynylene)

[0316] g. Polythiophene

[0317] h. Polypyrrol

[0318] i. Polyaniline

[0319] j. Poly(phenylene sulfide)

[0320] F) Cyanide

[0321] Y—O

[0322] A) Hydroxy, Alkoxy, etc.

[0323] a. Diol, triol, polyol, etc.

[0324] b. Cholesteryl group

[0325] c. Trisubstituted siloxy

[0326] B) Carboxylate

[0327] C) Phenoxy

[0328] D) Siloxy

[0329] E) Cyanate

[0330] F) Inorganic Oxides

[0331] Y—S

[0332] A) Thioalkyl

[0333] B) Thioaryl

[0334] C) Thiocyanate

[0335] D) Silylthio

[0336] Y—Si

[0337] A) Substituted silyl group

[0338] B) Tri-substituted silyl group with one or more functional groups

[0339] Y—N

[0340] A) Amino group (e.g., linear, branched, aromatic, or cyclic)

[0341] B) Mono and di-substituted amines

[0342] C) Imino group (e.g., linear, branched, aromatic, or cyclic)

[0343] D) Silylamino

[0344] FIGS. 10(a) and 10(b) show PL spectra of organic-terminated Siquantum dots, and FIGS. 11(a) and 11(b) show PL spectra oforganic-terminated Ge quantum dots. The Si and Ge quantum dots were madewith the methods described herein. The PL spectra show that the lightemission can be readily tuned from the red to the ultraviolet byexciting quantum dots of different sizes. The PL spectra are obtained byoptically exciting the quantum dots with wavelengths shorter than thewavelength at the absorption edge of the quantum dots. This lightemission is stable in ambient conditions. This stability is due in largepart to the relative completeness and stability of the surfacetermination, e.g., the surface termination and the interface between thecore and the surface termination is substantially defect free. In FIGS.10(a) and 10(b), the surfaces of the Si quantum dots are terminated with4-methoxyphenyl groups. In FIG. 10(b), the vertical axis represents anormalized photoluminescence signal from FIG. 10(a). In FIGS. 11(a) and11 (b), the surfaces of the Ge quantum dots are terminated with butylgroups. In FIG. 11(b), the vertical axis represents a normalizedphotoluminescence signal from FIG. 11(a). Similar results can be seenfor Si quantum dots having surfaces terminated with ethoxy groups and Gequantum dots having surfaces terminated with methyl groups.

[0345] The electronic and optical properties of these organic-terminatedSiQDs and GeQDs that are made in this fashion are unique in that theyshow size dependent properties that are uniquely consistent with quantumconfinement. The optical and electronic properties of these SiQDs andGeQDs are uniquely consistent with theoretical calculations moresophisticated than Effective Mass approaches, such as the EmpiricalPseudopotential Method and the Tight Binding Method. A comparison of thesize dependent energy gap calculated by these methods with measurementstaken on the SiQDs and GeQDs synthesized by the method disclosed hereinshow that the agreement is extremely good and is the best observed forany quantum dot formed of Si or Ge.

[0346] According to some embodiments of the invention, nanocompositematerials comprising quantum dots that are surface-terminated withvarious organic groups and dispersed in processible matrix materialssuch as organic polymers or sol-gels can exhibit new quantum phenomena.This new quantum phenomena in turn allow a large variety of newapplications (such as all-optical switching) and the fabrication ofdevice structures using low cost processing techniques (e.g., spincoating or dipping). Described herein are several novel syntheticschemes to fabricate these quantum dots and to functionalize theirsurfaces with molecular species that are chemically bonded to thesurface for stability and robustness. Use of such functionalized quantumdots avoids the need for expensive and specialized fabrication equipmentand facilities. The synthesis of these nanostructures can be readilyimplemented in many laboratories.

[0347] As discussed above, the value of this quantum dot nanostructurecan derive from molecular tethers serving multiple functions. Themolecular tethers may be active in a variety of ways, e.g.,electrically, chemically, mechanically, or optically active. Thisenables precise control of the electrical, optical, transport, chemical,and physical interactions between quantum dots and the surroundingmatrix material or the properties of individual quantum dots. Thesemolecular tethers can be a key innovation needed to develop new devicesand applications. Examples of particularly preferred embodiments ofoptically active molecular tethers are molecules with polarized orpolarizable sections or with large polarizabilities, donor-acceptormolecules, hetero-molecules, and charge transfer molecules.

[0348] Another major innovation comes from collective phenomenaresulting from nanocomposite materials that include coupled quantumsystems such as coupled quantum dots. The ability to attach activemolecular tethers to the quantum dot surface allows coupling quantumdots together in various one, two, and three-dimensional configurationsor arrays to initiate multiple quantum interference interactions betweenquantum dots that may be applied towards novel devices. The length andproperties of these molecular tethers can be tailored to enhance orgenerate specific quantum phenomena such as enhanced nonlinear opticalproperties. For instance, molecular tethers can provide charge transportbetween two or more interconnected quantum dots. For certain embodimentsof the present invention, the quantum dots can be massivelyinterconnected to an extent that is unlike previous efforts. Themassively interconnected quantum dot system can be comprised of morethan 2 interconnected quantum dots, preferably more than 10interconnected quantum dots, preferably more than 1000 interconnectedquantum dots, and most preferably more than 10⁹ interconnected quantumdots. For certain embodiments of the invention, the massivelyinterconnected quantum dot system can be comprised of two or moremassively interconnected quantum dot subsystems, which subsystems may ormay not be connected. The quantum dots can be interconnected via theformation of chemical bonds between appropriate molecular tethers ondifferent quantum dot surfaces. This, in turn, can be performed usingthe functionalization of the quantum dot surfaces as described earlierherein. After the quantum dot surface is functionalized, theinterconnection can proceed via chemical reaction between surfacefunctional groups, e.g., conjugated species, aromatics, etc. As a resultof such interconnection, a large variety of nanostructures is possible:

[0349] (1) n quantum dots coupled in a linear structure or array.

[0350] (2) n quantum dots coupled in an arbitrary 2-dimensionalstructure or array.

[0351] (3) n quantum dots coupled in an arbitrary 3-dimensionalstructure or array (e.g., to produce new lattice structure and newmaterials with tailorable properties.).

[0352] (4) n quantum dots attached to a polymer backbone to givecontrollable densities of quantum dots. These quantum dots can becoupled with other species (e.g., electron donating or acceptingmolecules) onto the polymer backbone to generate other new phenomena andapplications.

[0353] These nanostructures can have the properties of single quantumdots or an ensemble of quantum dots, which will be determined by thenature of the molecular tethers. This approach can be important forexploiting collective excitations in quantum dot systems towardsinnovative devices. These new nanostructures represent an importantinnovation in nanotechnology. Examples of particularly preferredembodiments of molecular tethers that can be used to interconnectquantum dots in this fashion and to generate controllable collectivephenomena include conjugated species such as conjugated polymers (e.g.,alkenes, alkynes, and aromatics).

[0354] The uniqueness of the synthetic process described above ismanifested in, but is not restricted to, the following properties of theresultant quantum dots and nanocomposite materials formed of suchquantum dots: (1) extremely large optical nonlinearities are manifested,e.g., in large values of Re[χ⁽³⁾ _(ijkl)], with values as high as 10⁻⁵cm²/W to 10 ⁻⁴ cm²/W. Previous materials with optical nonlinearities inthe infrared and more specifically in the important telecommunicationsregion of 1500 nm to 1600 nm typically have values of nonresonantdegenerate γ of ˜10⁻¹² cm²/W to 10⁻¹¹ cm²/W or less. As a result, theoptical nonlinearity, e.g. Re[χ⁽³⁾ _(ijkl)], of the quantum dots of thisinvention is ˜10⁶ to 10⁸ times larger than such previous materials; (2)stability of the quantum dots in a variety of environmental conditionsincluding ambient; (3) stability of the infrared to ultraviolet emissionin a variety of environmental conditions including ambient; (4) controlover the size of the quantum dots such that the light emission can besize-tuned from the infrared to the ultraviolet; (5) control over thesize of the quantum dots such that the nonlinear optical properties givelarge figures-of-merits that surpass those required for effectiveall-optical switching; (6) the nonlinear optical properties are suchthat all-optical switching occurs in a very short time (depending on thenature of the nonlinear optical mechanism (e.g., resonant ornon-resonant, the switching time can range from picoseconds to less than60 femtoseconds); (7) low switching energy (<<1×10⁻¹² Joules); (8)non-degenerate (e.g., control and data beams with different wavelengths)all-optical switching where the wavelengths of the relevant beams can bedetuned from each other over a very broad spectral range (>>100 nm) andstill maintain effective all-optical switching; (9) all-opticalswitching can occur throughout a broad wavelength range (e.g., from 400nm to 1600 nm).

[0355] Preferred Matrix Materials

[0356] According to some embodiments of the current invention, thematrix material that is used to host quantum dots can be selected from abroad range of materials due in large part to the versatile surfacetermination of the quantum dots as discussed above. These matrixmaterials can include, for example, organic and inorganic polymers orglasses with different properties including mechanical strength, opticaltransparency, lightwave transmissivity, thermal stability, dimensionalstability, low temperature flexibility, moisture absorption, andchemical inertness.

[0357] The matrix materials in some embodiments of the current inventionare preferred to be highly transparent and low absorption in thewavelength range from 600 nm to 2 μm. Also, they are preferred to behighly compatible with quantum dots so that a desired amount of quantumdots can be readily incorporated into the resulting nanocompositematerial without degrading optical and mechanical properties. Polymerswith special functional groups may be selected to facilitate solubilityinteractions and enhance compatibility with quantum dots. This isfurther explained as follows. Polymers with Lewis acid (base) functionalgroups, for example, can be selected to host quantum dots which aresurface-functionalized with Lewis base (acid) surface ligands. Anotherexample is to take advantage of hydrogen-bonding interactions. Polymersof hydrogen-bonding donors (acceptors) are the preferred matrixmaterials for quantum dots which are surface functionalized withhydrogen-bonding acceptors (donors). Additionally, polymers with strongdipolar groups are the preferred matrix materials for quantum dots whichare engineered with strong dipolar surface ligands. The strongintermolecular interactions described above greatly enhancecompatibility between the quantum dots and the matrix materials.Therefore, high contents of quantum dots can be readily incorporatedinto the matrix materials while maintaining desired uniformity andhomogeneity (e.g., allowing the quantum dots to be substantiallyuniformly dispersed throughout the matrix materials). Additionally,block copolymers can be used to further enhance compatibility byskillful selection of monomer units and block length. As a result, ananocomposite material comprising quantum dots and a matrix material canbe engineered to be of high optical quality and low scattering loss.More importantly, the linear and nonlinear index of refraction can betuned for a variety of applications primarily by adjusting the contentof the quantum dots and by selecting the matrix material, according tosome embodiments of the invention.

[0358] In addition to the optical properties and compatibility with thequantum dots, the preferred matrix material desirably should meet otherrequirements for a specific application. Thus, other properties can beconsidered in the selection of matrix materials.

[0359] One preferred matrix material is selected from a group ofpolymers with high glass transition temperature, T_(g), such aspolyimides, fluoropolymers (e.g., Teflon AF® brand fluoropolymersavailable from DuPont), polymers derived from B-stagedbisbenzocyclobutene monomers (e.g., Cyclotene® brand resins andCyclotene® brand fluorinated resins available from The Dow ChemicalCompany), phenolic resin, and fluorinated poly(aryl ether sulfide), forapplications where thermal stability is important.

[0360] Another preferred matrix material is selected from a group ofpolymers with low T_(g) such as poly(isobutylene),poly(diphenoxyphosphazene), and fluorinated acrylate (ZPU series fromZen Photonics Co., LTD) for applications where low temperatureflexibility and low birefringence are desired.

[0361] Another preferred matrix material can be selected fromphotosensitive polymers, such as fluoropolymers (e.g., Cytop® brandfluoropolymers available from Asahi), poly(methyl methacrylate), andphotoresists to facilitate lithographical fabrication of devices.

[0362] Another preferred matrix material is selected from a group ofcross-linkable polymers for applications where isotropic homogeneity ordimensional stability is required.

[0363] Another preferred matrix material is a blend of two or morepolymers which are engineered to tailor the optical and mechanicalproperties and thermal and chemical stability.

[0364] Another preferred matrix material is a copolymer including randomand block copolymer.

[0365] Another preferred matrix material is a homopolymer including, butnot limited to, the following:

[0366] Poly(vinyl alcohol)

[0367] Poly(vinyl butyral)-other

[0368] Poly(vinylcarbazol)

[0369] Poly(vinyl fluoride)

[0370] Poly methyl vinyl ether

[0371] Polyethylene

[0372] Polypropylene

[0373] Polystyrene

[0374] Poly(vinyl pyridine)

[0375] Polyimides

[0376] Poly(ethylene oxide)

[0377] Photoresist (positive or negative)

[0378] Cyclotene®

[0379] Fluorinated Cyclotene®

[0380] Cytop®

[0381] PMMA

[0382] Fluorinated acrylates

[0383] Poly(siloxanes)

[0384] Poly(silanes)

[0385] Poly(diphenoxyphosphazenes)

[0386] Poly(vinyl ferrocene)

[0387] Polycarbonate

[0388] Polystyrene

[0389] Poly(cyclic olefen) such as Zenor® and Zenex®

[0390] Teflon® AF®

[0391] Another preferred matrix material is a glass including, but notlimited to, the following:

[0392] Sol-gel derived glasses

[0393] Organically modified glasses

[0394] Spin-on glasses

[0395] Flow-glass

[0396] Dielectrics such as Low K FlowFill™ brand dielectrics of TrikonIndustries

[0397] Dielectrics such as Black Diamond™ brand dielectrics of AppliedMaterials, Inc.

[0398] Preferred Methods of Use

[0399] The engineered nonlinear nanocomposite materials of someembodiments of the present invention can be incorporated into an opticaldevice by a variety of methods, including a variety of standard methodsknown in the art. The flexibility to process the nanocomposite materialof embodiments of the current invention, desirably independent of thenonlinear optical properties, is a key benefit of embodiments of thecurrent invention. By selecting an appropriate matrix material andsolvent, engineered nonlinear nanocomposites can be deposited usingspin-coating, spin casting, dip coating, spraying, blade application,screen printing, and other methods commonly used in the process ofstandard semiconductor micro-fabrication.

[0400] While processes like spin-coating have been used in othercontexts, the combination of tuning the optical and mechanicalproperties of an engineered nonlinear nanocomposite material followed byspin-coating, or the like, is unlike previous capabilities. Traditionalnonlinear materials known in the art have chemical and mechanicalproperties that are directly linked to their optical properties. Theprocessing techniques that can be used to incorporate these materialsare therefore often limited to those that are compatible with thematerials themselves. For instance, LiNbO₃ is a crystal and cantherefore not be incorporated by spin coating.

[0401] The steps of incorporating a nonlinear nanocomposite materialinto a device by selecting desired optical properties, substantiallyindependently selecting desired chemical and/or mechanical properties tofacilitate incorporation by a particular technique (e.g., spin-coating),and then incorporating the nanocomposite material using that techniquerepresents a substantial improvement over previous incorporationmethods. While spin-coating has been discussed herein as a specificexample of a standard method of materials incorporation, this is donestrictly for exemplary purposes and should not be considered to limitthe scope of the invention.

[0402] For example, desired optical properties such as linear index ofrefraction and γ can be established by selecting or tuning at least oneof a chemical composition of quantum dot cores, a chemical compositionof quantum dot shells, a peak size of quantum dots, a thickness of theshells, a chemical composition of ligand layers, a chemical compositionof a matrix material, a concentration of the quantum dots in the matrixmaterial, and a degree of interconnection of the quantum dots (e.g.,using molecular tethers). Other desired optical properties such assingle-photon and multi-photon absorption characteristics can beestablished by selecting or tuning at least one of a chemicalcomposition of quantum dot cores, a chemical composition of quantum dotshells, a peak size of quantum dots, and a thickness of the shells.Desired chemical and mechanical properties can be established byselecting or tuning at least one of a chemical composition of ligandlayers and a chemical composition of a matrix material. As discussedpreviously, at least two of these desired properties can besubstantially independently established, according to some embodimentsof the invention.

[0403] In addition to standard incorporation techniques, other methodsof deposition such as layer-by-layer growth using polymers withalternating and complementary functionalities, as pioneered by GeroDecher and described in T. Sasaki et al., “Layer-by-Layer Assembly ofTitania Nanosheet/Polycation Composite Films,” Chem. Mater. 13, 4661(2001), the disclosure of which is incorporated herein by reference inits entirety, can be used to create films and coatings of laminatedlayer structures in the required thickness with desired density ofquantum dots.

[0404] All of the same processing techniques are also possible forquantum dot solids, including the ability to perform layer-by-layergrowth. Here again, the process of selecting the chemical properties ofthe surface ligands and solvent to facilitate incorporation by aparticular technique, desirably independent of the optical properties,represents a significant improvement over previous incorporationmethods.

[0405] The following provides some additional preferred methods ofincorporating an engineered nonlinear nanocomposite material into avariety of device.

[0406] The engineered nonlinear nanocomposite material can be dispersedin a polymer and subsequently dissolved in an appropriate solvent tocreate a fluid of sufficient viscosity to generate the desired thicknessof a film. The film thickness can be easily tailored by varying thesolvent content and therefore the viscosity. The specific quantum dotsurface chemistry is selected for compatibility with the selectedpolymer and solvent to be used. Some preferred materials include: DowChemical's Cyclotene®, which is B-stageddivinylsiloxane-bis-benzocyclobutene with Mesitylene and minor portionsof other organic compounds; poly (methyl methacrylate) (PMMA);photoresists (both positive and negative) used in semiconductormanufacturing; and so forth.

[0407] The engineered nonlinear nanocomposite material is dispersed in asuitable carrier fluid or solvent and applied evenly over the desiredsurface. Heat, vacuum, IR radiation, and/or an inert carrier gas arethen used to remove the carrier fluid, giving rise to a film of theengineered nonlinear nanocomposite material on the device.

[0408] The engineered nonlinear nanocomposite material is dispersed in acarrier gas, which is either reactive or inert. Appropriate carriergasses include, but are not limited to, SiH₄, N₂, H₂, O₂, and N₂O. Thegases are allowed to react under appropriate conditions of heat and/orplasma to cause a CVD film to be deposited on a substrate of choice. Inthis embodiment, a preferred substrate is a silicon wafer, optionallycomprising lithographic structures or patterns on the surface.

[0409] The engineered nonlinear nanocomposite material is incorporatedinto a sputter target, optionally using procedure (i) above.Alternatively, a pure target of a desired matrix material could be used(e.g., organic or inorganic targets, preferably SiO₂), and theengineered nonlinear nanocomposite material is introduced in a gas in asputter chamber. The engineered nonlinear nanocomposite material is thenincorporated directly into a growing sputtered film.

[0410] The engineered nonlinear nanocomposite material is heated andcaused to vaporize. The material vapors are then transported to adesired surface and condensed by keeping the surface at a suitabletemperature. The result is a solid film deposited on a device.

[0411] The same concepts can be used in systems that deposit Low Kmaterial such as Low K FlowFill™ brand dielectrics from TrikonIndustries or Black Diamond™ brand dielectrics from Applied Materials,Inc., thus incorporating quantum dots into low k films for even bettercontrol of the index of refraction and processability.

[0412] Preferred Nanocomposite Materials

[0413] Embodiments of the current invention comprise a nanocompositematerial with a controllable set of optical, mechanical, chemical, andelectronic properties. The nanocomposite material can comprise quantumdots dispersed in an organic and/or inorganic matrix material. Thematrix material may be either doped or undoped with molecular species,with a density of quantum dots therein such that the index of refraction(e.g., the linear index of refraction or the overall index ofrefraction) of the nanocomposite material falls between approximately1.3 and 5.0. Some embodiments of the nanocomposite material comprise atleast or more than 10% by weight of the quantum dots (e.g., at least 20%by weight of the quantum dots, at least 30% by weight of the quantumdots, at least 40% by weight of the quantum dots, or at least 50% byweight of the quantum dots, such as between approximately 50% and 60% byweight of the quantum dots). More particularly, some embodiments of thenanocomposite material can comprise the above discussed weightpercentages of the quantum dots with little or no agglomeration oraggregation of the quantum dots and with the quantum dots substantiallyuniformly dispersed throughout the matrix material. Also disclosed arenanocomposite materials (e.g., quantum dot solids) such that the densityof quantum dots within the nanocomposite material is betweenapproximately 0.005% and 75% by volume. Optionally, the index ofrefraction of the nanocomposite material can be additionally tuned byselecting a matrix material with a specific index of refraction and/orfurther doping the matrix material to modify that index. This providesadditional control over the optical characteristics. Optionally, thematrix material can be a polymerizable material with a desired index ofrefraction. The index of the refraction can be further fine-tuned bycross-linking via various activation mechanisms including thermal, photoillumination, plasma, and high energy radiations. The matrix material inwhich the quantum dots are dispersed may optionally have anintrinsically high χ⁽³⁾. The matrix material may optionally be anintrinsic matrix of a quantum dot solid.

[0414] The nanocomposite materials according to some embodiments arepreferably optically pure, with a homogeneous distribution of quantumdots dispersed therewithin. These quantum dots may be substantiallyuniformly dispersed as individual dots or as aggregates of controlledsizes (e.g., smaller aggregates up to massively interconnected quantumdot subsystems). The engineered nanocomposite materials are preferablyoptically homogeneous and uniform, so that little or no scatteringand/or mode disruption result from light passing through or past thematerial, as the specific application demands. For certain applications,close-packed micron- or sub-micron-sized clusters of quantum dotsdispersed in a polymer or other matrix material with a filling fractionoptimized to enhance local field effects may be preferable.

[0415] In addition, nanocomposite materials of some embodimentspreferably have an optical nonlinearity, such as Re[χ⁽³⁾ _(ijkl)]contributing to γ., e.g., under degenerate conditions, such asnonresonant degenerate conditions in a wavelength range-of-interest ofbetween approximately 10⁻¹² and 10⁻⁵ cm²/W, more preferably between10⁻¹⁰ and 10⁻⁵ cm²/W and most preferably between 10⁻⁸ and 10⁻⁵ cm²/W. Inparticular, certain embodiments of the nanocomposite material have γbeing at least 10⁻⁹ cm²/W (e.g., at least 10⁻⁸/cm²/W or at least 10⁻⁷cm²/W) when irradiated with light having a wavelength betweenapproximately 3×10⁻⁵ cm and 2×10⁻⁴ cm. More particularly, certainembodiments of the nanocomposite material have γ being at least 10⁻⁹cm²/W (e.g., at least 10⁻⁸ cm²/W or at least 10⁻⁷ cm²/W) when irradiatedwith light having a wavelength between approximately 1.25×10⁻⁴ cm and1.35×10⁻⁴ cm or between approximately 1.5×10⁻⁴ cm and 1.6×10⁻⁴. For γunder non-degenerate conditions, both relevant wavelengths (e.g.,wavelengths corresponding to trigger and data signals) can lie withinthe wavelength ranges stated above. Optical characteristics of thedisclosed nanocomposite materials can be evaluated in a variety ofconfigurations and are not restricted by the specific examples describedherein. One of skill in the art will appreciate that the linear andnonlinear optical properties of a material can be evaluated usingmethods such as Z-scan, FWM, cross-phase modulation, nonlinear phaseshift in an interferometer, nonlinear etalons, and so forth.

[0416] The mechanical properties of nanocomposite materials arepreferably selected to be compatible with incorporation into devicesselected from the list of: planar waveguides, nonplanar waveguides,optical fibers, waveguide cores, waveguide claddings, free-space optics,and hybrid optical devices. Such nanocomposite materials can be used ina variety of optical devices for switching, modulating, and manipulatinglight in ways such as for an optical switch, an optical cross-connect, awavelength converter, and the like, as well as combinations thereof.

[0417] The nanocomposite materials described herein can have a number ofkey attributes lacking in other materials. For instance, thenanocomposite materials can have an extremely large opticalnonlinearity. This optical nonlinearity can be represented by the realpart of various tensor elements of χ⁽³⁾, which include χ⁽³⁾ ₁₁₁₁, χ⁽³⁾₁₂₁₂, χ⁽³⁾ ₁₂₂₁, χ⁽³⁾ ₁₁₂₂, and various permutations of the energies ofthe optical fields involved, e.g., χ⁽³⁾ _(ijkl) (−ω₄, ω₁, ω₂, ω₃).According to some embodiments of the invention, the value of the realpart of these tensor elements falls in the range of 10⁻⁹ cm²/W to 10⁻⁴cm²/W. As a result, the nanocomposite materials allows all-opticaldevices to be made that can be effectively switched or controlled withvery low intensity light such as light from continuous wave laser diodesand also LEDs in some cases. This capability is highly sought after buthas not been previously achieved in a satisfactory manner.

[0418] Also, the nanocomposite materials described herein can exceedvarious FOM criteria for effective all-optical switching. In particular,certain embodiments of the nanocomposite material has a FOM that is atleast 1 (e.g., at least 1.5 or 1.8), where this particularfigure-of-merit can be defined as 2γ/βλ, where β is a two-photonabsorption coefficient of the nanocomposite material expressed in cm/W,and λ is a wavelength between approximately 3×10⁻⁵ cm and 2×10⁻⁴ cm,preferably between approximately 1.25×10⁻⁴ cm and 1.35×10⁻⁴ cm orbetween approximately 1.5×10⁻⁴ cm and 1.6×10⁻⁴. It should be recognizedthat other definitions for the FOM may be used instead. This isparticularly differentiating since, though other materials may possiblyhave large nonlinear refractive indices, the linear or nonlinear lossessuch as that originating from two photon absorption are oftensufficiently large so that the FOM is inadequate, the thermal propertiesare sufficiently poor such that the FOM is inferior, and the temporalresponse is considerably slower than with the nanocomposite materialsdescribed herein. Another important consequence is that significantlyshorter lengths of the nanocomposite materials described herein arerequired for effective all-optical switching devices. Thus,significantly smaller and faster devices can be made. As an example,rather than requiring centimeters or more of a conventional material toeffectively switch light, devices can be made with the nanocompositematerials with lengths of approximately ten microns to a fewmillimeters.

[0419] In addition, the nanocomposite materials described herein arerelatively simple and inexpensive to make, are more easily processed,are compatible with a large number of other material systems, and can beincorporated more readily into various device structure and in nearlyany device size. Epitaxial growth of the nanocomposite materialsdescribed herein is typically not required, which can be an advantagesince epitaxial growth is typically an expensive process that is oftennot amenable to simple processing or large area devices and is oftenincompatible with many other material systems (since it requiresepitaxial growth on material systems that are lattice-matched toitself). The nanocomposite materials can be deposited on varioussubstrates, substantially independently of their size, surface area, andsurface nature, in the form of films or coatings of varying thicknessesand can be formed into structures of various shapes and sizes.Importantly, these films, coatings, and structures made from thenanocomposite materials can be manufactured with a number of simple andinexpensive fabrication techniques such as spin coating, spray coating,doctor blading, and dip coating at ambient temperature and pressure, orusing conventional molding processes for engineering plastics andelastomers.

[0420] And, the nanocomposite materials described herein can haveoptical, chemical, thermal, and mechanical properties engineered to suitdevice or application requirements. For certain embodiments, thesevarious desirable attributes result in large part from the inherentflexibility in engineering the surface properties of quantum dots,substantially independently of their optical and electronic properties.In addition, these attributes can also result from use of organic orinorganic polymers with tailored optical, thermal, chemical, andmechanical properties suitable for different devices and applications.

[0421] Alternative Applications for Engineered Nanocomposite Materials

[0422] While the application of engineered nanocomposite materials as anonlinear material is described herein, it should be recognized thatsuch materials will also find applications in a variety of areas suchas, though not limited to, engineered resonant nonlinear nanocompositematerials, engineered linear nanocomposite materials, engineeredabsorptive nanocomposite materials, engineered electro-opticnanocomposite materials, engineered thermo-optic nanocompositematerials, engineered thermal nanocomposite materials, engineered gainnanocomposite materials, engineered magneto-optic nanocompositematerials, engineered magnetic nanocomposite materials, engineeredelectronic nanocomposite materials, engineered biological nanocompositematerials, engineered optoelectronic nanocomposite materials, andengineered mechanical nanocomposite materials. It should be recognizedthat the tunable physical, chemical, electronic, and optical propertiesof the materials described herein, as well as the methods ofincorporation thereof, can be used to create materials with specificcharacteristics tailored to many other applications.

[0423] At this point, one of ordinary skill in the art will recognizevarious advantages associated with some embodiments of the invention.Embodiments of this invention provide a method of synthesizingnanocrystalline materials. Embodiments of the invention also provide amethod of synthesis that produces quantum dots with organicallyfunctionalized surfaces and a method of synthesis that produces quantumdots with surfaces passivated with oxide. Embodiments of the inventionprovide a method of synthesis that is safe, energy efficient, scalable,and cost-effective. Embodiments of this invention also provide a methodof synthesis that employs environmentally benign starting materials orcommercially available or readily prepared materials. Advantageously,embodiments of this invention provide a method of synthesis that resultsin high yield and a method of synthesis that results in very soluble andprocessable products. Also, embodiments of the invention provide amethod of synthesis that results in highly crystalline material and thatyields quantum dots with a narrow size distribution, such as with adispersion of the size distribution is less than 15% rms. Embodiments ofthis invention provide a method of synthesis that yields quantum dotswith narrow shape distribution and a method of synthesis that producesquantum dots that are uniform in composition. In addition, embodimentsof the invention provide a method of synthesis that produces quantumdots that are uniform in surface chemistry.

[0424] Each of the patent applications, patents, publications, and otherpublished documents mentioned or referred to in this specification isherein incorporated by reference in its entirety, to the same extent asif each individual patent application, patent, publication, and otherpublished document was specifically and individually indicated to beincorporated by reference.

[0425] While the present invention has been described with reference tothe specific embodiments thereof, it should be understood by thoseskilled in the art that various changes may be made and equivalents maybe substituted without departing from the true spirit and scope of theinvention as defined by the appended claims. In addition, manymodifications may be made to adapt a particular situation, material,composition of matter, method, process step or steps, to the objective,spirit and scope of the present invention. All such modifications areintended to be within the scope of the claims appended hereto. Inparticular, while the methods disclosed herein have been described withreference to particular steps performed in a particular order, it willbe understood that these steps may be combined, sub-divided, orre-ordered to form an equivalent method without departing from theteachings of the present invention. Accordingly, unless specificallyindicated herein, the order and grouping of the steps is not alimitation of the present invention.

1. A quantum dot comprising: a) a core including a semiconductormaterial Y selected from the group consisting of Si and Ge; and b) ashell surrounding said core, said quantum dot being substantially defectfree such that said quantum dot exhibits photoluminescence with aquantum efficiency that is greater than 10 percent.
 2. The quantum dotof claim 1, wherein Y is Si, and said core has a diameter betweenapproximately 1 nm and 20 nm.
 3. The quantum dot of claim 1, wherein Yis Ge, and said core has a diameter between approximately 1 nm and 50nm.
 4. The quantum dot of claim 1, wherein said core has a diameterbetween approximately 1 nm and 10 nm.
 5. The quantum dot of claim 1,wherein said core is substantially spherical with an aspect ratiobetween approximately 0.8 and 1.2.
 6. The quantum dot of claim 1,wherein said shell has a thickness between approximately 0.1 nm and 5nm.
 7. The quantum dot of claim 1, wherein said shell includes an oxide.8. The quantum dot of claim 1, wherein said shell includes an oxideYO_(n) with n being between approximately 0 and
 2. 9. The quantum dot ofclaim 8, wherein n is between approximately 1.8 and
 2. 10. The quantumdot of claim 1, wherein said shell completely surrounds said core. 11.The quantum dot of claim 1, wherein said quantum dot exhibitsphotoluminescence with a quantum efficiency that is at least 20 percent.12. The quantum dot of claim 1, wherein said quantum dot exhibitsphotoluminescence with a quantum efficiency that is at least 50 percent.13-21. (Canceled)